diff --git a/app/api/admin/meta/route.ts b/app/api/admin/meta/route.ts
index 73ed1aa..ed4ea5a 100644
--- a/app/api/admin/meta/route.ts
+++ b/app/api/admin/meta/route.ts
@@ -1,13 +1,12 @@
 // app/api/admin/meta/route.ts
 import { NextResponse } from 'next/server';
 import { prisma } from '@/lib/prisma';
-import { getServerSession } from 'next-auth';
-import { authOptions } from '@/lib/auth-options';
+import { adminGuard } from '@/lib/admin-check';
 
 export async function GET() {
-  // Basic protection: ensure signed-in user is admin. Modify as needed.
-  // If you want stricter, get session from context or check roles.
-  // Here we skip session check to let dev access; but production should check.
+  const denied = await adminGuard();
+  if (denied) return denied;
+
   try {
     const courses = await prisma.course.findMany({ orderBy: { title: 'asc' } });
     const playlists = await prisma.playlist.findMany({ orderBy: { title: 'asc' } });
diff --git a/app/api/enrollments/sync/route.ts b/app/api/enrollments/sync/route.ts
index 767657a..c882f12 100644
--- a/app/api/enrollments/sync/route.ts
+++ b/app/api/enrollments/sync/route.ts
@@ -1,122 +1,78 @@
 // app/api/enrollments/sync/route.ts
 import { NextResponse } from 'next/server';
-import type { NextRequest } from 'next/server';
 import { getServerSession } from 'next-auth';
 import { authOptions } from '@/lib/auth-options';
 import { prisma } from '@/lib/prisma';
+import { normalizeEmail } from '@/lib/normalize-email';
 
-export async function POST(req: NextRequest) {
-  console.log('🔥 [SYNC-ENROLLMENTS] POST endpoint called');
-  console.log('🔥 [SYNC-ENROLLMENTS] Request method:', req.method);
-  console.log('🔥 [SYNC-ENROLLMENTS] Request URL:', req.url);
-  
+/**
+ * Enrols the signed-in user in the courses listed for them on the allow-list.
+ * The course codes are read from the database; anything the browser sends is
+ * ignored, so a user cannot enrol themselves in a course they were not given.
+ */
+export async function POST() {
   try {
     const session = await getServerSession(authOptions);
-    console.log('🔥 [SYNC-ENROLLMENTS] Session check:', !!session?.user?.email);
-    console.log('🔥 [SYNC-ENROLLMENTS] Session user:', session?.user);
-    
-    if (!session?.user?.email) {
-      console.log('🔥 [SYNC-ENROLLMENTS] No session or email, returning 401');
+    const email = normalizeEmail(session?.user?.email);
+    if (!email) {
       return NextResponse.json({ error: 'Unauthorized' }, { status: 401 });
     }
 
-    const userEmail = session.user.email;
-    console.log('🔥 [SYNC-ENROLLMENTS] About to parse request body...');
-    
-    const body = await req.json();
-    console.log('🔥 [SYNC-ENROLLMENTS] Raw body received:', JSON.stringify(body, null, 2));
-    
-    const levels: string[] = body?.levels || [];
-    console.log('🔥 [SYNC-ENROLLMENTS] Extracted levels:', levels);
-    console.log('🔥 [SYNC-ENROLLMENTS] Levels type check - isArray:', Array.isArray(levels));
-    console.log('🔥 [SYNC-ENROLLMENTS] Levels length:', levels.length);
-
-    console.log('🔥 [SYNC-ENROLLMENTS] User:', userEmail);
-    console.log('🔥 [SYNC-ENROLLMENTS] Received levels:', levels);
-
-    if (!Array.isArray(levels) || levels.length === 0) {
-      console.log('🔥 [SYNC-ENROLLMENTS] VALIDATION FAILED!');
-      console.log('🔥 [SYNC-ENROLLMENTS] levels isArray:', Array.isArray(levels));
-      console.log('🔥 [SYNC-ENROLLMENTS] levels length:', levels?.length);
-      console.log('🔥 [SYNC-ENROLLMENTS] levels value:', levels);
-      console.log('🔥 [SYNC-ENROLLMENTS] Returning 400 - Levels array required');
-      return NextResponse.json(
-        { error: 'Levels array required' },
-        { status: 400 }
-      );
-    }
-
-    // Get user
-    const user = await prisma.user.findUnique({
-      where: { email: userEmail },
+    const user = await prisma.user.findFirst({
+      where: { email: { equals: email, mode: 'insensitive' } },
+      select: { id: true },
     });
     if (!user) {
-      console.log('[SYNC-ENROLLMENTS] User not found in database:', userEmail);
       return NextResponse.json({ error: 'User not found' }, { status: 404 });
     }
 
-    console.log('[SYNC-ENROLLMENTS] Found user:', user.id, user.email);
+    const allowedStudent = await prisma.allowedStudent.findFirst({
+      where: { email: { equals: email, mode: 'insensitive' } },
+      select: { levels: true, active: true },
+    });
 
-    const createdEnrollments = [];
+    const levels = allowedStudent?.active
+      ? allowedStudent.levels
+          .split(',')
+          .map((level) => level.trim())
+          .filter((level) => level.length > 0)
+      : [];
 
-    // For each course level, find the course and create enrollment
-    for (const level of levels) {
-      try {
-        console.log('[SYNC-ENROLLMENTS] Looking for course with code:', level);
-        
-        const course = await prisma.course.findUnique({
-          where: { code: level },
-        });
-
-        if (!course) {
-          console.warn(`[SYNC-ENROLLMENTS] Course code not found: ${level}`);
-          continue;
-        }
-
-        console.log('[SYNC-ENROLLMENTS] Found course:', course.id, course.code, course.title);
-
-        // Check if enrollment already exists
-        const existing = await prisma.enrollment.findFirst({
-          where: { userId: user.id, courseId: course.id },
-        });
-
-        if (existing) {
-          console.log('[SYNC-ENROLLMENTS] Enrollment already exists:', existing.id);
-        } else {
-          const enrollment = await prisma.enrollment.create({
-            data: {
-              userId: user.id,
-              courseId: course.id,
-            },
-          });
-          console.log('[SYNC-ENROLLMENTS] Created new enrollment:', enrollment.id);
-          createdEnrollments.push({
-            courseCode: level,
-            courseId: course.id,
-            enrollmentId: enrollment.id,
-          });
-        }
-      } catch (err) {
-        console.error(`[SYNC-ENROLLMENTS] Failed to create enrollment for level ${level}:`, err);
-      }
+    if (levels.length === 0) {
+      return NextResponse.json({ message: 'Enrollments synced', created: [] });
     }
 
-    console.log('[SYNC-ENROLLMENTS] Summary - Created enrollments:', createdEnrollments.length);
-    console.log('[SYNC-ENROLLMENTS] Details:', createdEnrollments);
+    const courses = await prisma.course.findMany({
+      where: { code: { in: levels } },
+      select: { id: true, code: true },
+    });
 
-    return NextResponse.json(
-      {
-        message: 'Enrollments synced',
-        created: createdEnrollments,
-      },
-      { status: 200 }
-    );
+    const missing = levels.filter((level) => !courses.some((c) => c.code === level));
+    if (missing.length > 0) {
+      console.warn(`[SYNC-ENROLLMENTS] Course codes not found: ${missing.join(', ')}`);
+    }
+
+    const created = [];
+    for (const course of courses) {
+      const existing = await prisma.enrollment.findUnique({
+        where: { userId_courseId: { userId: user.id, courseId: course.id } },
+        select: { id: true },
+      });
+      if (existing) continue;
+
+      const enrollment = await prisma.enrollment.create({
+        data: { userId: user.id, courseId: course.id },
+      });
+      created.push({
+        courseCode: course.code,
+        courseId: course.id,
+        enrollmentId: enrollment.id,
+      });
+    }
+
+    return NextResponse.json({ message: 'Enrollments synced', created });
   } catch (err) {
-    console.error('🔥 [SYNC-ENROLLMENTS] FATAL ERROR:', err);
-    console.error('🔥 [SYNC-ENROLLMENTS] Error stack:', err instanceof Error ? err.stack : 'No stack');
-    return NextResponse.json(
-      { error: 'Failed to sync enrollments' },
-      { status: 500 }
-    );
+    console.error('[SYNC-ENROLLMENTS] error:', err);
+    return NextResponse.json({ error: 'Failed to sync enrollments' }, { status: 500 });
   }
 }
diff --git a/app/api/playlists/[id]/route.ts b/app/api/playlists/[id]/route.ts
index 5b7acb8..20b405a 100644
--- a/app/api/playlists/[id]/route.ts
+++ b/app/api/playlists/[id]/route.ts
@@ -3,6 +3,7 @@ import { NextResponse } from "next/server";
 import { prisma } from "../../../../lib/prisma";
 import { getServerSession } from "next-auth";
 import { authOptions } from "../../../../lib/auth-options";
+import { filterAccessibleVideos } from "../../../../lib/watch-progress";
 
 export async function GET(req: Request, context: any) {
   try {
@@ -20,6 +21,7 @@ export async function GET(req: Request, context: any) {
 
     const user = await prisma.user.findUnique({
       where: { email: session.user.email },
+      include: { enrollments: { select: { courseId: true } } },
     });
     if (!user) {
       return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
@@ -37,21 +39,31 @@ export async function GET(req: Request, context: any) {
                 image: true,
               },
             },
+            videoCourses: { select: { courseId: true, exclusive: true } },
           },
           orderBy: { index: "asc" },
         },
         course: true,
+        courses: { select: { courseId: true } },
       },
     });
 
     if (!playlist) return NextResponse.json({ error: "Not found" }, { status: 404 });
 
-    const enrolled = await prisma.enrollment.findUnique({
-      where: { userId_courseId: { userId: user.id, courseId: playlist.courseId } },
-    });
-    if (!enrolled) return NextResponse.json({ error: "Forbidden" }, { status: 403 });
+    // Enrolment in the primary course or any course the playlist is shared with.
+    const userCourseIds = user.enrollments.map((e) => e.courseId);
+    const playlistCourseIds = [playlist.courseId, ...playlist.courses.map((c) => c.courseId)];
+    if (!playlistCourseIds.some((courseId) => userCourseIds.includes(courseId))) {
+      return NextResponse.json({ error: "Forbidden" }, { status: 403 });
+    }
 
-    return NextResponse.json({ playlist });
+    // Hide videos reserved for courses this user is not in, as /api/playlists does.
+    const visible = {
+      ...playlist,
+      videos: filterAccessibleVideos(playlist.videos, userCourseIds),
+    };
+
+    return NextResponse.json({ playlist: visible });
   } catch (err: any) {
     console.error("GET /api/playlists/[id] error:", err);
     return NextResponse.json({ error: "Server error" }, { status: 500 });
diff --git a/app/api/progress/route.ts b/app/api/progress/route.ts
index 6690037..a6020c3 100644
--- a/app/api/progress/route.ts
+++ b/app/api/progress/route.ts
@@ -3,6 +3,14 @@ import { NextResponse } from 'next/server';
 import { getServerSession } from 'next-auth';
 import { authOptions } from '@/lib/auth-options';
 import { prisma } from '@/lib/prisma';
+import {
+  UNLOCK_THRESHOLD_PERCENT,
+  mergeRanges,
+  sanitizeRanges,
+  totalSeconds,
+  unlockNextVideo,
+  type SecRange,
+} from '@/lib/watch-progress';
 
 export async function GET(req: Request) {
   try {
@@ -47,36 +55,6 @@ export async function GET(req: Request) {
   }
 }
 
-// Helper function to calculate unique watched seconds from segments
-function calculateWatchedSeconds(segments: Array<{ startSec: number; endSec: number }>): number {
-  if (segments.length === 0) return 0;
-  
-  // Sort and merge overlapping ranges
-  const sorted = segments.sort((a, b) => a.startSec - b.startSec);
-  const merged: Array<[number, number]> = [];
-  
-  for (const seg of sorted) {
-    if (merged.length === 0) {
-      merged.push([seg.startSec, seg.endSec]);
-    } else {
-      const last = merged[merged.length - 1];
-      if (seg.startSec <= last[1] + 0.5) {
-        // Overlapping or adjacent, merge
-        last[1] = Math.max(last[1], seg.endSec);
-      } else {
-        // Gap, new range
-        merged.push([seg.startSec, seg.endSec]);
-      }
-    }
-  }
-  
-  let total = 0;
-  for (const [start, end] of merged) {
-    total += Math.max(0, end - start);
-  }
-  return Math.round(total);
-}
-
 export async function POST(req: Request) {
   try {
     const session = await getServerSession(authOptions);
@@ -86,137 +64,131 @@ export async function POST(req: Request) {
     const userEmail = session.user.email;
 
     const body = await req.json().catch(() => ({}));
-    const { videoId, playlistId, watchedSec, lastPos, duration } = body ?? {};
+    const { videoId, watchedSec, lastPos, duration } = body ?? {};
 
-    if (!videoId)
+    if (!videoId || typeof videoId !== 'string')
       return NextResponse.json({ error: 'videoId required' }, { status: 400 });
-    if (!duration || Number(duration) <= 0)
-      return NextResponse.json({ error: 'duration required' }, { status: 400 });
-
-    const watched = Number(watchedSec ?? 0);
-    const lastPosition = Number(lastPos ?? 0);
-    const dur = Number(duration);
 
     // look up user id
-    const user = await prisma.user.findUnique({ where: { email: userEmail } });
+    const user = await prisma.user.findUnique({
+      where: { email: userEmail },
+      select: { id: true },
+    });
     if (!user)
       return NextResponse.json({ error: 'user not found' }, { status: 404 });
 
     // Validate that the video exists
-    const video = await prisma.video.findUnique({ where: { id: videoId } });
+    const video = await prisma.video.findUnique({
+      where: { id: videoId },
+      select: { id: true, playlistId: true, durationSec: true },
+    });
     if (!video) {
       // Log at debug level since this is expected when users have old cached references
       console.debug(`[PROGRESS] Video not found: ${videoId} (likely deleted video with cached browser reference)`);
       return NextResponse.json({ error: 'video not found' }, { status: 404 });
     }
 
-    // Ensure VideoProgress exists or create it
-    let videoProgress = await prisma.videoProgress.findUnique({
-      where: { userId_videoId: { userId: user.id, videoId } },
-    });
+    // The stored duration wins; the player's value is only a fallback for
+    // videos uploaded without one.
+    const dur =
+      video.durationSec && video.durationSec > 0 ? video.durationSec : Number(duration);
+    if (!Number.isFinite(dur) || dur <= 0)
+      return NextResponse.json({ error: 'duration required' }, { status: 400 });
 
-    if (!videoProgress) {
-      videoProgress = await prisma.videoProgress.create({
-        data: {
+    const lastPosition = Math.max(0, Math.floor(Number(lastPos ?? 0)) || 0);
+
+    // Current players send the ranges actually played. Tabs still running the
+    // previous player only send a total, from which one range is inferred.
+    let incoming: SecRange[];
+    if (Array.isArray(body?.ranges)) {
+      incoming = sanitizeRanges(body.ranges, dur);
+    } else {
+      const watched = Number(watchedSec ?? 0);
+      incoming = sanitizeRanges(
+        watched > 0 ? [[lastPosition - watched, lastPosition]] : [],
+        dur
+      );
+    }
+
+    const where = { userId_videoId: { userId: user.id, videoId } };
+
+    const { progress, segments } = await prisma.$transaction(async (tx) => {
+      const existingProgress = await tx.videoProgress.upsert({
+        where,
+        update: {},
+        create: {
           userId: user.id,
           videoId,
           watchedSec: 0,
           lastPos: 0,
           percent: 0,
-          durationSec: dur,
+          durationSec: Math.round(dur),
         },
       });
-    }
 
-    // Record the watch segment if watched seconds > 0
-    let newSegment = null;
-    if (watched > 0) {
-      newSegment = await prisma.videoWatchSegment.create({
-        data: {
-          userId: user.id,
-          videoId,
-          startSec: Math.max(0, lastPosition - watched),
-          endSec: lastPosition,
-        },
+      const existing = await tx.videoWatchSegment.findMany({
+        where: { userId: user.id, videoId },
+        select: { startSec: true, endSec: true, watchedAt: true },
+        orderBy: { startSec: 'asc' },
       });
-    }
 
-    // Fetch all segments for this user+video to recalculate totals
-    const allSegments = await prisma.videoWatchSegment.findMany({
-      where: { userId: user.id, videoId },
-      select: { startSec: true, endSec: true },
-      orderBy: { createdAt: 'asc' },
-    });
-
-    // Calculate total unique watched seconds
-    const totalWatchedSec = calculateWatchedSeconds(allSegments);
-
-    // Calculate completion percentage
-    const ratio = dur > 0 ? totalWatchedSec / dur : 0;
-    const percentInt = Math.min(100, Math.round(ratio * 100));
-    const completed = percentInt >= 80; // Changed from 90 to 80 for unlock threshold
-
-    // Update VideoProgress with recalculated values
-    const upserted = await prisma.videoProgress.update({
-      where: { userId_videoId: { userId: user.id, videoId } },
-      data: {
-        watchedSec: totalWatchedSec,
-        lastPos: Math.max(videoProgress.lastPos ?? 0, lastPosition),
-        percent: percentInt,
-        durationSec: dur,
-        completed,
-        updatedAt: new Date(),
-      },
-    });
-
-    // If 80% watched, unlock next video in playlist for this user
-    let unlockedNext = null;
-    if (completed && playlistId) {
-      // find current video
-      const current = await prisma.video.findUnique({ where: { id: videoId } });
-      if (current && current.playlistId === playlistId) {
-        // Find next video that is NOT instant access and NOT globally locked
-        // Skip any instant access videos in the sequence
-        const nextVideos = await prisma.video.findMany({
-          where: {
-            playlistId,
-            index: { gt: current.index },
-            locked: false,
-            instantAccess: false,
-          },
-          orderBy: { index: 'asc' },
-          take: 1,
+      // Keep one row per distinct watched stretch instead of one per update.
+      const now = new Date();
+      const merged = mergeRanges([
+        ...existing,
+        ...incoming.map((r) => ({ ...r, watchedAt: now })),
+      ]);
+      const unchanged =
+        merged.length === existing.length &&
+        merged.every(
+          (m, i) => m.startSec === existing[i].startSec && m.endSec === existing[i].endSec
+        );
+      if (!unchanged) {
+        await tx.videoWatchSegment.deleteMany({ where: { userId: user.id, videoId } });
+        await tx.videoWatchSegment.createMany({
+          data: merged.map((m) => ({
+            userId: user.id,
+            videoId,
+            startSec: m.startSec,
+            endSec: m.endSec,
+            watchedAt: m.watchedAt ?? now,
+          })),
         });
-
-        if (nextVideos.length > 0) {
-          const next = nextVideos[0];
-          // Create a VideoUnlock record for this user (per-user unlock tracking)
-          const unlock = await prisma.videoUnlock.upsert({
-            where: { userId_videoId: { userId: user.id, videoId: next.id } },
-            update: {}, // if already exists, do nothing
-            create: { userId: user.id, videoId: next.id },
-          });
-          unlockedNext = { id: next.id, title: next.title };
-        }
       }
-    }
 
-    return NextResponse.json({ ok: true, progress: upserted, unlockedNext, segment: newSegment });
+      // Progress never goes backwards, even if segment rows were pruned.
+      const mergedSec = totalSeconds(merged);
+      const percent = Math.max(
+        existingProgress.percent,
+        Math.min(100, Math.round((mergedSec / dur) * 100))
+      );
+      const completed =
+        existingProgress.completed || percent >= UNLOCK_THRESHOLD_PERCENT;
+
+      const updated = await tx.videoProgress.update({
+        where,
+        data: {
+          watchedSec: Math.max(existingProgress.watchedSec, mergedSec),
+          lastPos: Math.max(existingProgress.lastPos ?? 0, lastPosition),
+          percent,
+          durationSec: Math.round(dur),
+          completed,
+          updatedAt: now,
+        },
+      });
+
+      return { progress: updated, segments: merged };
+    });
+
+    // Completing a video unlocks the next one in its playlist for this user.
+    const unlockedNext = progress.completed
+      ? await unlockNextVideo(user.id, video)
+      : null;
+
+    return NextResponse.json({ ok: true, progress, unlockedNext, segments });
   } catch (err: any) {
     console.error('progress error', err);
-    
-    // Handle foreign key constraint violations
-    if (err.code === 'P2003') {
-      const constraint = err.meta?.constraint_name;
-      if (constraint?.includes('videoId')) {
-        console.error(`[PROGRESS] Foreign key violation - invalid videoId: ${err.meta}`);
-        return NextResponse.json(
-          { error: 'Invalid video reference' },
-          { status: 400 }
-        );
-      }
-    }
-    
+
     return NextResponse.json(
       { error: err?.message ?? 'server error' },
       { status: 500 }
diff --git a/app/api/thumbnails/[...path]/route.ts b/app/api/thumbnails/[...path]/route.ts
index 0607c94..73543a9 100644
--- a/app/api/thumbnails/[...path]/route.ts
+++ b/app/api/thumbnails/[...path]/route.ts
@@ -4,6 +4,7 @@
 import { NextResponse } from 'next/server';
 import * as fs from 'fs';
 import * as path from 'path';
+import { getViewer } from '@/lib/video-access';
 
 // Path to thumbnails directory (should match UPLOADS_DIR/thumbnails)
 const THUMBNAILS_DIR = path.join(process.env.UPLOADS_DIR || '/uploads', 'thumbnails');
@@ -23,6 +24,11 @@ export async function GET(
       );
     }
 
+    // Thumbnails show course content, so they need a signed-in user.
+    if (!(await getViewer())) {
+      return NextResponse.json({ error: 'Unauthorized' }, { status: 401 });
+    }
+
     // Construct the file path and validate it
     const requestedPath = path.join(THUMBNAILS_DIR, ...pathSegments);
     
@@ -44,7 +50,7 @@ export async function GET(
     }
 
     // Read the file
-    const fileContent = fs.readFileSync(requestedPath);
+    const fileContent = await fs.promises.readFile(requestedPath);
 
     // Determine content type based on file extension
     let contentType = 'application/octet-stream';
@@ -64,8 +70,7 @@ export async function GET(
       status: 200,
       headers: {
         'Content-Type': contentType,
-        'Cache-Control': 'public, max-age=31536000, immutable', // Cache for 1 year (thumbnails don't change)
-        'Access-Control-Allow-Origin': '*', // Allow CORS if needed
+        'Cache-Control': 'private, max-age=31536000, immutable', // Cache for 1 year (thumbnails don't change)
       },
     });
   } catch (error) {
@@ -90,6 +95,8 @@ export async function HEAD(
       return new NextResponse(null, { status: 400 });
     }
 
+    if (!(await getViewer())) return new NextResponse(null, { status: 401 });
+
     const requestedPath = path.join(THUMBNAILS_DIR, ...pathSegments);
     
     // Security: prevent directory traversal attacks
@@ -107,7 +114,7 @@ export async function HEAD(
       status: 200,
       headers: {
         'Content-Type': 'image/jpeg', // Default for HEAD requests
-        'Cache-Control': 'public, max-age=31536000, immutable',
+        'Cache-Control': 'private, max-age=31536000, immutable',
       },
     });
   } catch (error) {
diff --git a/app/api/user/unlocks/route.ts b/app/api/user/unlocks/route.ts
index 490b5cf..558b150 100644
--- a/app/api/user/unlocks/route.ts
+++ b/app/api/user/unlocks/route.ts
@@ -3,6 +3,7 @@ import { NextResponse } from 'next/server';
 import { getServerSession } from 'next-auth';
 import { authOptions } from '@/lib/auth-options';
 import { prisma } from '@/lib/prisma';
+import { checkUnlockEligibility } from '@/lib/watch-progress';
 
 export async function GET(req: Request) {
   try {
@@ -59,40 +60,31 @@ export async function POST(req: Request) {
     if (!user)
       return NextResponse.json({ error: 'user not found' }, { status: 404 });
 
-    // Verify video exists
-    const video = await prisma.video.findUnique({
-      where: { id: videoId },
-      select: { id: true, locked: true, instantAccess: true },
-    });
-
-    if (!video) {
-      return NextResponse.json({ error: 'video not found' }, { status: 404 });
+    // Admins can open anything; everyone else must have earned the unlock.
+    const role = (session.user as any).role;
+    const isAdmin = role === 'admin' || role === 'superadmin';
+    if (!isAdmin) {
+      const check = await checkUnlockEligibility(user.id, videoId);
+      if (!check.ok) {
+        return NextResponse.json({ error: check.error }, { status: check.status });
+      }
+      if (check.free) {
+        return NextResponse.json({ message: 'Video does not need unlocking', unlock: null });
+      }
+    } else {
+      const exists = await prisma.video.findUnique({ where: { id: videoId }, select: { id: true } });
+      if (!exists) {
+        return NextResponse.json({ error: 'video not found' }, { status: 404 });
+      }
     }
 
-    // Check if already unlocked
-    const existingUnlock = await prisma.videoUnlock.findUnique({
+    const unlock = await prisma.videoUnlock.upsert({
       where: { userId_videoId: { userId: user.id, videoId } },
+      update: {},
+      create: { userId: user.id, videoId },
     });
 
-    if (existingUnlock) {
-      return NextResponse.json({ 
-        message: 'Video already unlocked',
-        unlock: existingUnlock 
-      });
-    }
-
-    // Create the unlock record
-    const unlock = await prisma.videoUnlock.create({
-      data: {
-        userId: user.id,
-        videoId: videoId,
-      },
-    });
-
-    return NextResponse.json({ 
-      message: 'Video unlocked successfully',
-      unlock 
-    });
+    return NextResponse.json({ message: 'Video unlocked successfully', unlock });
 
   } catch (err: any) {
     console.error('user unlocks POST error', err);
diff --git a/app/api/videos/[id]/route.ts b/app/api/videos/[id]/route.ts
index a807688..4b9db98 100644
--- a/app/api/videos/[id]/route.ts
+++ b/app/api/videos/[id]/route.ts
@@ -4,6 +4,7 @@ import { prisma } from "../../../../lib/prisma";
 import { getServerSession } from "next-auth";
 import { authOptions } from "../../../../lib/auth-options";
 import { getVideoUrls } from "../../../../lib/video-urls";
+import { checkVideoAccess } from "../../../../lib/video-access";
 
 export async function GET(req: Request, context: any) {
   try {
@@ -74,6 +75,16 @@ export async function GET(req: Request, context: any) {
       return NextResponse.json({ error: "Forbidden" }, { status: 403 });
     }
 
+    // Sequential locks apply to students even when they follow a direct link.
+    const role = (session.user as any).role;
+    const isAdmin = role === "admin" || role === "superadmin";
+    if (!isAdmin) {
+      const access = await checkVideoAccess({ id: user.id, isAdmin: false }, id);
+      if (!access.ok) {
+        return NextResponse.json({ error: access.error }, { status: access.status });
+      }
+    }
+
     // Find the next video (index = current.index + 1)
     const next = await prisma.video.findFirst({
       where: {
diff --git a/app/api/videos/hls/[...path]/route.ts b/app/api/videos/hls/[...path]/route.ts
index 543a134..cf13296 100644
--- a/app/api/videos/hls/[...path]/route.ts
+++ b/app/api/videos/hls/[...path]/route.ts
@@ -4,6 +4,7 @@
 import { NextResponse } from 'next/server';
 import * as fs from 'fs';
 import * as path from 'path';
+import { checkVideoAccess, getViewer } from '@/lib/video-access';
 
 // Path to HLS directory (should match UPLOADS_DIR/hls)
 const HLS_DIR = path.join(process.env.UPLOADS_DIR || '/uploads', 'hls');
@@ -23,6 +24,17 @@ export async function GET(
       );
     }
 
+    // Signed-in viewers only, and only for videos they may watch.
+    // The first path segment is the video id.
+    const viewer = await getViewer();
+    if (!viewer) {
+      return NextResponse.json({ error: 'Unauthorized' }, { status: 401 });
+    }
+    const access = await checkVideoAccess(viewer, pathSegments[0]);
+    if (!access.ok) {
+      return NextResponse.json({ error: access.error }, { status: access.status });
+    }
+
     // Construct the file path and validate it
     const requestedPath = path.join(HLS_DIR, ...pathSegments);
     
@@ -44,7 +56,7 @@ export async function GET(
     }
 
     // Read the file
-    const fileContent = fs.readFileSync(requestedPath);
+    const fileContent = await fs.promises.readFile(requestedPath);
 
     // Determine content type based on file extension
     let contentType = 'application/octet-stream';
@@ -62,8 +74,7 @@ export async function GET(
       status: 200,
       headers: {
         'Content-Type': contentType,
-        'Cache-Control': 'public, max-age=3600', // Cache for 1 hour
-        'Access-Control-Allow-Origin': '*', // Allow CORS if needed
+        'Cache-Control': 'private, max-age=3600', // Viewer's browser only, 1 hour
         'Accept-Ranges': 'bytes',
       },
     });
@@ -89,6 +100,11 @@ export async function HEAD(
       return new NextResponse(null, { status: 400 });
     }
 
+    const viewer = await getViewer();
+    if (!viewer) return new NextResponse(null, { status: 401 });
+    const access = await checkVideoAccess(viewer, pathSegments[0]);
+    if (!access.ok) return new NextResponse(null, { status: access.status });
+
     const requestedPath = path.join(HLS_DIR, ...pathSegments);
     
     // Security: prevent directory traversal attacks
diff --git a/app/dashboard/dashboard-client.tsx b/app/dashboard/dashboard-client.tsx
index 1965b75..e5d82e2 100644
--- a/app/dashboard/dashboard-client.tsx
+++ b/app/dashboard/dashboard-client.tsx
@@ -400,7 +400,7 @@ export default function DashboardClient() {
                                             handleOpenVideo(pl.id, v.id);
                                             return;
                                           }
-                                          if ((v as any).index === 0) {
+                                          if (v.id === pl.videos?.[0]?.id) {
                                             handleOpenVideo(pl.id, v.id);
                                             return;
                                           }
@@ -417,7 +417,7 @@ export default function DashboardClient() {
                                             className="w-full h-full object-cover"
                                             loading="lazy"
                                           />
-                                          {(v as any).locked || (!(v as any).instantAccess && (v as any).index !== 0 && !userUnlocks.has(v.id)) ? (
+                                          {(v as any).locked || (!(v as any).instantAccess && v.id !== pl.videos?.[0]?.id && !userUnlocks.has(v.id)) ? (
                                             <div className="absolute inset-0 bg-black/40 flex items-center justify-center">
                                               <Lock className="w-5 h-5 text-white" />
                                             </div>
diff --git a/app/uploads/videos/[...path]/route.ts b/app/uploads/videos/[...path]/route.ts
index 45f8cf5..1114661 100644
--- a/app/uploads/videos/[...path]/route.ts
+++ b/app/uploads/videos/[...path]/route.ts
@@ -8,6 +8,8 @@ import { NextResponse } from "next/server";
 import fs from "fs";
 import path from "path";
 import { Readable } from "stream";
+import { prisma } from "@/lib/prisma";
+import { checkVideoAccess, getViewer } from "@/lib/video-access";
 
 const VIDEOS_DIR = path.join(process.env.UPLOADS_DIR || "/uploads", "videos");
 
@@ -63,12 +65,52 @@ function parseRange(
   return { start, end };
 }
 
+// Maps a requested file to the Video row it belongs to, then applies the same
+// access rule as the video API. Files with no Video row are admin-only.
+const VIDEO_ID_TTL_MS = 5 * 60_000;
+const videoIdByPath = new Map<string, { expires: number; videoId: string | null }>();
+
+async function findVideoId(segments: string[]): Promise<string | null> {
+  const relative = segments.join("/");
+  const hit = videoIdByPath.get(relative);
+  if (hit && hit.expires > Date.now()) return hit.videoId;
+
+  const byUrl = await prisma.video.findFirst({
+    where: { url: `/uploads/videos/${relative}` },
+    select: { id: true },
+  });
+  // Uploads are normally stored as {videoId}.mp4
+  const byName =
+    byUrl ??
+    (await prisma.video.findUnique({
+      where: { id: path.basename(relative, path.extname(relative)) },
+      select: { id: true },
+    }));
+
+  const videoId = byName?.id ?? null;
+  if (videoIdByPath.size >= 5000) videoIdByPath.clear();
+  videoIdByPath.set(relative, { expires: Date.now() + VIDEO_ID_TTL_MS, videoId });
+  return videoId;
+}
+
+async function authorize(segments: string[]): Promise<{ status: number; error: string } | null> {
+  const viewer = await getViewer();
+  if (!viewer) return { status: 401, error: "Unauthorized" };
+  if (viewer.isAdmin) return null;
+
+  const videoId = await findVideoId(segments);
+  if (!videoId) return { status: 404, error: "Not found" };
+
+  const access = await checkVideoAccess(viewer, videoId);
+  return access.ok ? null : { status: access.status, error: access.error };
+}
+
 function baseHeaders(filePath: string, stats: fs.Stats): Record<string, string> {
   const ext = path.extname(filePath).toLowerCase();
   return {
     "Content-Type": CONTENT_TYPES[ext] ?? "application/octet-stream",
     "Accept-Ranges": "bytes",
-    "Cache-Control": "public, max-age=3600",
+    "Cache-Control": "private, max-age=3600",
     "Last-Modified": stats.mtime.toUTCString(),
   };
 }
@@ -84,6 +126,11 @@ export async function GET(
       return NextResponse.json({ error: "Invalid request" }, { status: 400 });
     }
 
+    const denied = await authorize(params?.path || []);
+    if (denied) {
+      return NextResponse.json({ error: denied.error }, { status: denied.status });
+    }
+
     const stats = await statFile(filePath);
     if (!stats) {
       console.log(`[Videos] File not found: ${filePath}`);
@@ -132,6 +179,9 @@ export async function HEAD(
     const filePath = resolveVideoPath(params?.path || []);
     if (!filePath) return new NextResponse(null, { status: 400 });
 
+    const denied = await authorize(params?.path || []);
+    if (denied) return new NextResponse(null, { status: denied.status });
+
     const stats = await statFile(filePath);
     if (!stats) return new NextResponse(null, { status: 404 });
 
diff --git a/app/videoplayer/page.tsx b/app/videoplayer/page.tsx
index aa9e700..ff65a05 100644
--- a/app/videoplayer/page.tsx
+++ b/app/videoplayer/page.tsx
@@ -27,6 +27,7 @@ import { Button } from '@/components/ui/button';
 import { HlsPlayer } from '@/components/hls';
 import { useSession } from 'next-auth/react';
 import Image from 'next/image';
+import { toast } from 'sonner';
 
 type VideoFromApi = {
   id: string;
@@ -50,261 +51,256 @@ type VideoFromApi = {
   } | null;
 };
 
+type ProgressResult = {
+  progress?: { percent?: number };
+  segments?: WatchSegment[];
+  unlockedNext?: { id: string; title: string } | null;
+};
+
+type TrackedVideo = { videoId: string; durationSec: number | null };
+
+/**
+ * Records which parts of a video were actually played and reports them to
+ * /api/progress.
+ *
+ * It samples the <video> element on a timer rather than relying on media
+ * events, so it keeps working when the element is swapped (one per video) or
+ * when playback starts before anything else on the page has settled. A sample
+ * only counts as watched when the playhead moved forward by no more than the
+ * wall-clock time that passed, which excludes seeks and scrubbing.
+ */
 export function useWatchTracker(
   videoRef: React.RefObject<HTMLVideoElement | null>,
-  opts: { sendIntervalMs?: number; commitIntervalSec?: number; debug?: boolean } = {}
+  opts: {
+    sendIntervalMs?: number;
+    sampleIntervalMs?: number;
+    onResult?: (videoId: string, result: ProgressResult) => void;
+  } = {}
 ) {
   const sendIntervalMs = opts.sendIntervalMs ?? 5000;
-  const commitIntervalSec = opts.commitIntervalSec ?? 3;
-  const debug = !!opts.debug;
+  const sampleIntervalMs = opts.sampleIntervalMs ?? 500;
 
-  const rangesRef = React.useRef<Array<[number, number]>>([]);
-  const lastTimeRef = React.useRef<number | null>(null);
-  const currentRangeStartRef = React.useRef<number | null>(null);
-  const sendTimerRef = React.useRef<number | null>(null);
-  const commitTimerRef = React.useRef<number | null>(null);
-
-  // merging helper
-  function mergeRanges(ranges: Array<[number, number]>) { 
-    if (!ranges.length) return [];
-    ranges.sort((a, b) => a[0] - b[0]);
-    const merged: Array<[number, number]> = [];
-    for (const [s, e] of ranges) {
-      if (!merged.length) merged.push([s, e]);
-      else {
-        const last = merged[merged.length - 1];
-        if (s <= last[1] + 0.5) last[1] = Math.max(last[1], e);
-        else merged.push([s, e]);
-      }
-    }
-    return merged;
-  }
-
-  function addRange(s: number, e: number) {
-    if (e <= s) return;
-    if (debug) console.debug("[tracker] addRange", s, e);
-    rangesRef.current.push([s, e]);
-    rangesRef.current = mergeRanges(rangesRef.current);
-  }
-
-  function getUniqueWatchedSec() {
-    let sum = 0;
-    for (const [a, b] of rangesRef.current) sum += Math.max(0, b - a);
-    return Math.round(sum);
-  }
-
-  // commit the "current" playing segment to ranges (useful while playing)
-  function commitCurrentRange() {
-    const el = videoRef.current;
-    if (!el) return;
-    const start = currentRangeStartRef.current;
-    const now = el.currentTime;
-    if (start === null) return;
-    // only commit if we've moved forward at least 0.5s to avoid noise
-    if (now - start >= 0.5) {
-      addRange(start, now);
-      // keep currentRangeStart open at 'now' so we continue accumulating
-      currentRangeStartRef.current = now;
-    }
-  }
-
-  // send progress to server (same signature you had)
-  async function sendProgress(args: { videoId: string; playlistId?: string; duration: number; extra?: Record<string, any> }) {
-    const { videoId, playlistId, duration, extra } = args;
-    const watchedSec = getUniqueWatchedSec();
-    const el = videoRef.current;
-    const lastPos = Math.floor(el?.currentTime ?? 0);
-
-    if (!duration || duration <= 0) return null;
-    if (watchedSec === 0 && (el?.paused ?? true)) return null;
-
-    const payload = { videoId, playlistId, watchedSec, lastPos, duration, ...extra };
-    try {
-      if (debug) console.debug("[tracker] sendProgress", payload);
-      const res = await fetch("/api/progress", {
-        method: "POST",
-        headers: { "Content-Type": "application/json" },
-        body: JSON.stringify(payload),
-      });
-      
-      // If video not found, stop trying to track progress
-      if (!res.ok) {
-        if (res.status === 404) {
-          console.warn(`[tracker] Video ${videoId} not found, stopping progress tracking`);
-          stopAutoSend(); // Stop automatic progress tracking for deleted video
-          return null;
-        }
-        return null;
-      }
-      
-      return res.ok ? await res.json().catch(() => null) : null;
-    } catch (err) {
-      if (debug) console.warn("[tracker] sendProgress error", err);
-      return null;
-    }
-  }
-
-  // start/stop auto-send
-  function startAutoSend(args: { videoId: string; playlistId?: string; duration: number }) {
-    stopAutoSend(); // clear existing
-
-    // immediate send (best-effort)
-    sendProgress(args).catch(() => {});
-
-    // periodic send
-    sendTimerRef.current = window.setInterval(() => {
-      const watched = getUniqueWatchedSec();
-      const el = videoRef.current;
-      const isPlaying = !!(el && !el.paused && !el.ended);
-      if (watched > 0 || isPlaying) {
-        sendProgress(args).catch(() => {});
-      }
-    }, sendIntervalMs) as unknown as number;
-
-    // commit currently playing ranges periodically (so continuous play is captured)
-    commitTimerRef.current = window.setInterval(() => {
-      commitCurrentRange();
-    }, commitIntervalSec * 1000) as unknown as number;
-
-    if (debug) console.debug("[tracker] startAutoSend", { sendIntervalMs, commitIntervalSec });
-  }
-
-  function stopAutoSend(opts?: { sendFinal?: boolean; videoId?: string; playlistId?: string; duration?: number }) {
-    if (sendTimerRef.current) {
-      window.clearInterval(sendTimerRef.current);
-      sendTimerRef.current = null;
-    }
-    if (commitTimerRef.current) {
-      window.clearInterval(commitTimerRef.current);
-      commitTimerRef.current = null;
-    }
-    if (opts?.sendFinal && opts.videoId && opts.duration) {
-      sendProgress({ videoId: opts.videoId, playlistId: opts.playlistId, duration: opts.duration }).catch(() => {});
-    }
-    if (debug) console.debug("[tracker] stopAutoSend");
-  }
-
-  // attach listeners to populate rangesRef
+  const onResultRef = React.useRef(opts.onResult);
   React.useEffect(() => {
+    onResultRef.current = opts.onResult;
+  });
+
+  const activeRef = React.useRef<TrackedVideo | null>(null);
+  const rangesRef = React.useRef<Array<[number, number]>>([]);
+  const lastSampleRef = React.useRef<{ el: HTMLVideoElement; time: number; wall: number } | null>(null);
+  const lastPosRef = React.useRef(0);
+  const elementDurationRef = React.useRef(0);
+  const dirtyRef = React.useRef(false);
+
+  const reset = React.useCallback((next: TrackedVideo | null) => {
+    activeRef.current = next;
+    rangesRef.current = [];
+    lastSampleRef.current = null;
+    lastPosRef.current = 0;
+    elementDurationRef.current = 0;
+    dirtyRef.current = false;
+  }, []);
+
+  const sample = React.useCallback(() => {
     const el = videoRef.current;
-    if (!el) {
-      if (debug) console.debug("[tracker] no video element to attach");
+    if (!activeRef.current || !el || typeof el.currentTime !== 'number') {
+      lastSampleRef.current = null;
       return;
     }
 
-    currentRangeStartRef.current = null;
-    lastTimeRef.current = el.currentTime ?? 0;
+    const prev = lastSampleRef.current;
+    const now = el.currentTime;
+    const wall = performance.now();
+    lastSampleRef.current = { el, time: now, wall };
 
-    const onPlay = () => {
-      currentRangeStartRef.current = el.currentTime;
-      lastTimeRef.current = el.currentTime;
-      if (debug) console.debug("[tracker] play", currentRangeStartRef.current);
-    };
+    // First sample of a new element: nothing to compare against yet.
+    if (!prev || prev.el !== el) return;
 
-    const onPause = () => {
-      if (currentRangeStartRef.current !== null) {
-        addRange(currentRangeStartRef.current, el.currentTime);
-        currentRangeStartRef.current = null;
-      }
-      if (debug) console.debug("[tracker] pause, ranges:", rangesRef.current);
-    };
+    lastPosRef.current = now;
+    if (Number.isFinite(el.duration) && el.duration > 0) {
+      elementDurationRef.current = el.duration;
+    }
+    if (el.seeking) return;
 
-    const onTimeUpdate = () => {
-      const now = el.currentTime;
-      const last = lastTimeRef.current ?? now;
-      // detect seek (big jump)
-      if (Math.abs(now - last) > 2.5) {
-        if (currentRangeStartRef.current !== null) {
-          addRange(currentRangeStartRef.current, last);
+    const delta = now - prev.time;
+    if (delta <= 0) return; // paused, or jumped backwards
+
+    // Real playback cannot outrun the clock (timers may fire late in
+    // background tabs, hence wall-clock time rather than the interval).
+    const elapsedSec = (wall - prev.wall) / 1000;
+    const maxAdvance = elapsedSec * Math.max(1, el.playbackRate || 1) + 0.75;
+    if (delta > maxAdvance) return; // jumped forwards
+
+    const ranges = rangesRef.current;
+    const last = ranges[ranges.length - 1];
+    if (last && Math.abs(last[1] - prev.time) < 0.01) {
+      last[1] = now; // still in the same stretch of playback
+    } else {
+      ranges.push([prev.time, now]);
+    }
+    dirtyRef.current = true;
+  }, [videoRef]);
+
+  const flush = React.useCallback(async (keepalive = false) => {
+    const active = activeRef.current;
+    if (!active || !dirtyRef.current || rangesRef.current.length === 0) return;
+
+    const duration = active.durationSec || Math.floor(elementDurationRef.current);
+    if (!duration || duration <= 0) return; // metadata not loaded yet; try again next tick
+
+    // Everything played this session is resent each time; the server merges,
+    // so a dropped request loses nothing.
+    const ranges = rangesRef.current.map(([s, e]) => [s, e]);
+    dirtyRef.current = false;
+
+    try {
+      const res = await fetch('/api/progress', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        keepalive,
+        body: JSON.stringify({
+          videoId: active.videoId,
+          ranges,
+          lastPos: Math.floor(lastPosRef.current),
+          duration,
+        }),
+      });
+
+      const stillActive = activeRef.current?.videoId === active.videoId;
+      if (!res.ok) {
+        if (res.status === 404) {
+          // Video was deleted: stop tracking it.
+          if (stillActive) reset(null);
+        } else if (stillActive) {
+          dirtyRef.current = true;
         }
-        currentRangeStartRef.current = now;
-        if (debug) console.debug("[tracker] seek detected, start:", now);
+        return;
       }
-      lastTimeRef.current = now;
-      // we do not addRange here to avoid flooding; commit timer handles mid-play commits
-    };
 
-    const onEnded = () => {
-      if (currentRangeStartRef.current !== null) {
-        addRange(currentRangeStartRef.current, el.duration ?? lastTimeRef.current ?? 0);
-        currentRangeStartRef.current = null;
-      }
-      if (debug) console.debug("[tracker] ended, ranges:", rangesRef.current);
-    };
+      const json = await res.json().catch(() => null);
+      if (json) onResultRef.current?.(active.videoId, json);
+    } catch {
+      if (activeRef.current?.videoId === active.videoId) dirtyRef.current = true;
+    }
+  }, [reset]);
 
-    el.addEventListener("play", onPlay);
-    el.addEventListener("pause", onPause);
-    el.addEventListener("timeupdate", onTimeUpdate);
-    el.addEventListener("ended", onEnded);
+  /** Begin tracking a video. Anything recorded for a previous video is dropped. */
+  const start = React.useCallback((video: TrackedVideo) => reset(video), [reset]);
+
+  /** Send what is left for the current video, then stop tracking it. */
+  const stop = React.useCallback(() => {
+    void flush(true);
+    reset(null);
+  }, [flush, reset]);
+
+  React.useEffect(() => {
+    const sampleTimer = window.setInterval(sample, sampleIntervalMs);
+    const sendTimer = window.setInterval(() => void flush(), sendIntervalMs);
+
+    // Leaving or hiding the page: get the latest progress out while we can.
+    const flushNow = () => {
+      sample();
+      void flush(true);
+    };
+    const onVisibility = () => {
+      if (document.visibilityState === 'hidden') flushNow();
+    };
+    document.addEventListener('visibilitychange', onVisibility);
+    window.addEventListener('pagehide', flushNow);
 
     return () => {
-      el.removeEventListener("play", onPlay);
-      el.removeEventListener("pause", onPause);
-      el.removeEventListener("timeupdate", onTimeUpdate);
-      el.removeEventListener("ended", onEnded);
-      stopAutoSend();
+      window.clearInterval(sampleTimer);
+      window.clearInterval(sendTimer);
+      document.removeEventListener('visibilitychange', onVisibility);
+      window.removeEventListener('pagehide', flushNow);
     };
-    // eslint-disable-next-line react-hooks/exhaustive-deps
-  }, [videoRef.current]);
+  }, [sample, flush, sampleIntervalMs, sendIntervalMs]);
 
-  return {
-    getUniqueWatchedSec,
-    addRange,
-    sendProgress,
-    startAutoSend,
-    stopAutoSend,
-  };
+  return { start, stop };
 }
 
 
 export default function Page() {
   const search = useSearchParams();
   const router = useRouter();
-  const playlistId = search.get('playlistId') ?? undefined;
+  const playlistIdParam = search.get('playlistId') ?? undefined;
   const videoId = search.get('videoId') ?? undefined;
 
-  const { playlist, isLoading: playlistLoading } = usePlaylist(playlistId);
-  const { video, next, isLoading: videoLoading } = useVideo(videoId);
+  const { video, isLoading: videoLoading, error: videoError } = useVideo(videoId);
+  // Links from watch history and liked videos carry no playlist; use the video's own.
+  const { playlist, isLoading: playlistLoading } = usePlaylist(
+    playlistIdParam ?? (video as any)?.playlistId ?? undefined
+  );
 
   // Segments state for current video and playlist videos
   const [currentSegments, setCurrentSegments] = React.useState<WatchSegment[]>([]);
   const [playlistSegments, setPlaylistSegments] = React.useState<Record<string, WatchSegment[]>>({});
   const [currentProgress, setCurrentProgress] = React.useState(0);
-  
+
   // Per-user unlock state
   const [userUnlocks, setUserUnlocks] = React.useState<Set<string>>(new Set()); // Set of unlocked videoIds
-  const [videoInstantAccess, setVideoInstantAccess] = React.useState<Record<string, boolean>>({}); // videoId -> instantAccess
   const [unlockingVideo, setUnlockingVideo] = React.useState<string | null>(null); // videoId being unlocked
 
-  // inside your videoplayer component
-const videoRef = React.useRef<HTMLVideoElement | null>(null);
-const tracker = useWatchTracker(videoRef, { sendIntervalMs: 5000 });
+  const videoRef = React.useRef<HTMLVideoElement | null>(null);
+  const trackedVideoIdRef = React.useRef<string | null>(null);
+  const userUnlocksRef = React.useRef(userUnlocks);
+  React.useEffect(() => {
+    userUnlocksRef.current = userUnlocks;
+  }, [userUnlocks]);
 
-// Fetch segments for current video
+  // Every progress update returns the server's view: percentage, merged
+  // segments, and the next video if this one just unlocked it.
+  const handleProgressResult = React.useCallback((id: string, result: ProgressResult) => {
+    const segments = result.segments;
+    if (segments) {
+      setPlaylistSegments((prev) => ({ ...prev, [id]: segments }));
+    }
+    if (id === trackedVideoIdRef.current) {
+      if (segments) setCurrentSegments(segments);
+      if (typeof result.progress?.percent === 'number') setCurrentProgress(result.progress.percent);
+    }
+    const unlocked = result.unlockedNext;
+    if (unlocked && !userUnlocksRef.current.has(unlocked.id)) {
+      setUserUnlocks((prev) => new Set(prev).add(unlocked.id));
+      toast.success(`Unlocked: ${unlocked.title}`);
+    }
+  }, []);
+
+  const tracker = useWatchTracker(videoRef, { onResult: handleProgressResult });
+  const { start: startTracking, stop: stopTracking } = tracker;
+
+// Load saved progress for the current video. Live updates then arrive with
+// each progress save, so there is no need to poll.
 React.useEffect(() => {
   if (!videoId) return;
+  let cancelled = false;
 
   const fetchSegments = async () => {
     try {
-      const res = await fetch(`/api/progress/segments?videoId=${videoId}`);
+      const [res, progressRes] = await Promise.all([
+        fetch(`/api/progress/segments?videoId=${videoId}`),
+        fetch(`/api/progress?videoId=${videoId}`),
+      ]);
+      if (cancelled) return;
       if (res.ok) {
         const data = await res.json();
-        setCurrentSegments(data.segments ?? []);
+        if (!cancelled) setCurrentSegments(data.segments ?? []);
       }
-      const progressRes = await fetch(`/api/progress?videoId=${videoId}`);
       if (progressRes.ok) {
         const data = await progressRes.json();
-        setCurrentProgress(data.percent ?? 0);
+        if (!cancelled) setCurrentProgress(data.percent ?? 0);
       }
     } catch (err) {
       console.error('Failed to fetch segments:', err);
     }
   };
 
+  setCurrentSegments([]);
+  setCurrentProgress(0);
   fetchSegments();
-  // Refresh segments every 5 seconds
-  const interval = setInterval(fetchSegments, 5000);
-  return () => clearInterval(interval);
+  return () => {
+    cancelled = true;
+  };
 }, [videoId]);
 
 // Fetch segments for all playlist videos
@@ -313,10 +309,8 @@ React.useEffect(() => {
 
   const fetchPlaylistSegments = async () => {
     const segments: Record<string, WatchSegment[]> = {};
-    const instantAccess: Record<string, boolean> = {};
-    
+
     for (const video of playlist.videos) {
-      // Fetch segments
       try {
         const res = await fetch(`/api/progress/segments?videoId=${video.id}`);
         if (res.ok) {
@@ -326,15 +320,13 @@ React.useEffect(() => {
       } catch (err) {
         console.error(`Failed to fetch segments for video ${video.id}:`, err);
       }
-      
-      // Track instantAccess status from video object
-      instantAccess[video.id] = (video as any).instantAccess ?? false;
     }
-    
-    setPlaylistSegments(segments);
-    setVideoInstantAccess(instantAccess);
+
+    // Keep anything newer that arrived from a progress save in the meantime.
+    setPlaylistSegments((prev) => ({ ...segments, ...prev }));
   };
 
+  setPlaylistSegments({});
   fetchPlaylistSegments();
 }, [playlist?.videos]);
 
@@ -345,8 +337,9 @@ React.useEffect(() => {
       const res = await fetch('/api/user/unlocks');
       if (res.ok) {
         const data = await res.json();
-        const unlockedVideoIds = new Set((data.unlocks?.map((u: any) => u.videoId) ?? []) as string[]);
-        setUserUnlocks(unlockedVideoIds);
+        const unlockedVideoIds = (data.unlocks?.map((u: any) => u.videoId) ?? []) as string[];
+        // Merge, in case a progress save unlocked something while this loaded.
+        setUserUnlocks((prev) => new Set([...prev, ...unlockedVideoIds]));
       }
     } catch (err) {
       console.error('Failed to fetch user unlocks:', err);
@@ -356,55 +349,20 @@ React.useEffect(() => {
   fetchUserUnlocks();
 }, []);
 
+// Track whichever video is loaded; switching videos flushes the old one first.
+const loadedVideoId = video?.id ? String(video.id) : null;
+const loadedVideoDuration: number | null = video?.durationSec ?? null;
 React.useEffect(() => {
-  if (!video) return;
+  if (!loadedVideoId) return;
 
-  let didCancel = false;
-  let poll: number | null = null;
-
-  const startWhenReady = () => {
-    if (didCancel) return;
-    const el = videoRef.current;
-    const duration = (video.durationSec ?? Math.floor(el?.duration ?? 0)) || 0;
-
-    if (el) {
-      tracker.startAutoSend({
-        videoId: String(video.id),
-        playlistId: playlist?.id,
-        duration,
-      });
-    } else {
-      // poll until the video element mounts (should be quick)
-      poll = window.setInterval(() => {
-        if (videoRef.current) {
-          if (poll) {
-            window.clearInterval(poll);
-            poll = null;
-          }
-          tracker.startAutoSend({
-            videoId: String(video.id),
-            playlistId: playlist?.id,
-            duration: (video.durationSec ?? Math.floor(videoRef.current?.duration ?? 0)) || 0,
-          });
-        }
-      }, 150) as unknown as number;
-    }
-  };
-
-  startWhenReady();
+  trackedVideoIdRef.current = loadedVideoId;
+  startTracking({ videoId: loadedVideoId, durationSec: loadedVideoDuration });
 
   return () => {
-    didCancel = true;
-    if (poll) {
-      window.clearInterval(poll);
-      poll = null;
-    }
-    const duration = video?.durationSec ?? Math.floor(videoRef.current?.duration ?? 0);
-    // send final snapshot
-    tracker.stopAutoSend({ sendFinal: true, videoId: String(video?.id ?? ''), playlistId: playlist?.id, duration });
+    stopTracking();
+    trackedVideoIdRef.current = null;
   };
-// eslint-disable-next-line react-hooks/exhaustive-deps
-}, [video?.id, playlist?.id]);
+}, [loadedVideoId, loadedVideoDuration, startTracking, stopTracking]);
 
 
   const current: VideoFromApi = (video as VideoFromApi) ?? {
@@ -425,84 +383,40 @@ React.useEffect(() => {
     return `${mins}:${secs}`;
   };
 
+  // A video is open when it is the first in the playlist, marked instant
+  // access, or unlocked for this user. The lecturer's lock overrides all three.
+  const firstVideoId: string | undefined = playlist?.videos?.[0]?.id;
+  const isAccessible = (v: VideoFromApi) =>
+    !v.locked &&
+    (!!(v as any).instantAccess || v.id === firstVideoId || userUnlocks.has(v.id));
+
+  const openVideo = (v: VideoFromApi) => {
+    router.push(`/videoplayer?playlistId=${playlist?.id}&videoId=${v.id}`);
+  };
+
+  // The server decides whether the unlock is earned and explains when it is not.
   const handleUnlock = async (v: VideoFromApi) => {
-    console.log('unlock request for', v.id);
-    
-    // Prevent multiple unlock attempts
-    if (unlockingVideo === v.id) return;
-    
+    if (unlockingVideo === v.id || v.locked) return;
+
     setUnlockingVideo(v.id);
-    
     try {
-      // Can't unlock if globally locked
-      if ((v as any).locked) {
-        console.log('Video is globally locked, cannot unlock');
-        return;
-      }
-      
-      // Can't unlock if already unlocked or has instant access
-      if ((v as any).instantAccess || userUnlocks.has(v.id)) {
-        console.log('Video already accessible');
-        router.push(`/videoplayer?playlistId=${playlist?.id}&videoId=${v.id}`);
-        return;
-      }
-      
-      // Check if previous video in sequence is completed
-      if (!playlist?.videos) return;
-      
-      const videoIndex = (v as any).index;
-      if (videoIndex <= 0) {
-        // First video should always be accessible
-        router.push(`/videoplayer?playlistId=${playlist?.id}&videoId=${v.id}`);
-        return;
-      }
-      
-      // Find previous video
-      const previousVideo = playlist.videos.find((pv: any) => pv.index === videoIndex - 1);
-      if (!previousVideo) {
-        console.log('Previous video not found');
-        return;
-      }
-      
-      // Check if previous video is completed
-      const progressRes = await fetch(`/api/progress?videoId=${previousVideo.id}`);
-      if (!progressRes.ok) {
-        console.error('Failed to fetch previous video progress');
-        alert('Error checking unlock requirements. Please try again.');
-        return;
-      }
-      
-      const progressData = await progressRes.json();
-      const isCompleted = progressData.percent >= 90; // Consider 90% as completed
-      
-      if (!isCompleted) {
-        console.log('Previous video not completed yet - need', Math.ceil(90 - progressData.percent), '% more');
-        // TODO: Show toast notification explaining unlock requirements
-        alert(`You need to complete ${Math.ceil(90 - progressData.percent)}% more of the previous video to unlock this one.`);
-        return;
-      }
-      
-      // Previous video is completed, unlock this video
-      const unlockRes = await fetch('/api/user/unlocks', {
+      const res = await fetch('/api/user/unlocks', {
         method: 'POST',
         headers: { 'Content-Type': 'application/json' },
         body: JSON.stringify({ videoId: v.id }),
       });
-      
-      if (unlockRes.ok) {
-        // Update local state
-        setUserUnlocks(prev => new Set(prev).add(v.id));
-        console.log('Video unlocked successfully');
-        // Redirect to the unlocked video
-        router.push(`/videoplayer?playlistId=${playlist?.id}&videoId=${v.id}`);
-      } else {
-        const errorData = await unlockRes.json().catch(() => ({ error: 'Unknown error' }));
-        console.error('Failed to unlock video:', errorData.error);
-        alert(`Failed to unlock video: ${errorData.error}`);
+      const data = await res.json().catch(() => ({}));
+
+      if (!res.ok) {
+        toast.error(data?.error ?? 'Could not unlock this video. Please try again.');
+        return;
       }
+
+      setUserUnlocks((prev) => new Set(prev).add(v.id));
+      openVideo(v);
     } catch (err) {
-      console.error('Error checking unlock conditions:', err);
-      alert('Error checking if video can be unlocked. Please try again.');
+      console.error('Error unlocking video:', err);
+      toast.error('Could not unlock this video. Please try again.');
     } finally {
       setUnlockingVideo(null);
     }
@@ -585,7 +499,11 @@ React.useEffect(() => {
                         ref={videoRef}
                       />
                     ) : (
-                      <div className="flex items-center justify-center h-80">Video not available</div>
+                      <div className="flex items-center justify-center h-80 px-6 text-center">
+                        {videoError?.message && !/^HTTP \d+$/.test(videoError.message)
+                          ? videoError.message
+                          : 'Video not available'}
+                      </div>
                     )}
                   </div>
                 </CardContent>
@@ -698,7 +616,7 @@ React.useEffect(() => {
                 ) : (
                   playlist?.videos?.map((v: VideoFromApi) => {
                     const active = String(v.id) === String(current.id);
-                    const watchedPercent = 0; // Replace with actual watched % from your state/API
+                    const accessible = isAccessible(v);
                     return (
                       <div
                         key={v.id}
@@ -707,29 +625,13 @@ React.useEffect(() => {
                           active ? 'border-primary bg-muted' : 'border-transparent hover:border-border'
                         }`}
                         onClick={() => {
-                          // Check if video is accessible based on the logic:
-                          // 1. If locked at schema level, always deny
-                          if ((v as any).locked) return;
-                          // 2. If instantAccess, always allow
-                          if ((v as any).instantAccess) {
-                            router.push(`/videoplayer?playlistId=${playlist.id}&videoId=${v.id}`);
-                            return;
-                          }
-                          // 3. If first video (index 0), always allow
-                          if ((v as any).index === 0) {
-                            router.push(`/videoplayer?playlistId=${playlist.id}&videoId=${v.id}`);
-                            return;
-                          }
-                          // 4. Otherwise, check if user has unlocked it
-                          if (userUnlocks.has(v.id)) {
-                            router.push(`/videoplayer?playlistId=${playlist.id}&videoId=${v.id}`);
-                          }
+                          if (accessible) openVideo(v);
                         }}
                       >
                         <div className="flex items-center gap-3">
                           <div className="relative w-20 h-12 shrink-0 overflow-hidden rounded">
                             <img src={v.thumbnail ?? '/01.jpg'} alt={v.title} className="w-full h-full object-cover" loading="lazy" />
-                            {(v as any).locked || (!(v as any).instantAccess && (v as any).index !== 0 && !userUnlocks.has(v.id)) ? (
+                            {!accessible ? (
                               <div className="absolute inset-0 bg-black/40 flex items-center justify-center">
                                 <Lock className="w-5 h-5 text-white" />
                               </div>
@@ -742,28 +644,28 @@ React.useEffect(() => {
                               <Badge>{fmt(v)}</Badge>
                             </div>
                             <div className="text-xs text-muted-foreground truncate mt-1">
-                              {(v as any).locked ? 'Locked' : (v as any).instantAccess || (v as any).index === 0 ? 'Available' : userUnlocks.has(v.id) ? 'Available' : active ? 'Now playing' : 'Locked'}
+                              {active ? 'Now playing' : accessible ? 'Available' : 'Locked'}
                             </div>
                           </div>
-                          
 
-                          {(v as any).locked || (!(v as any).instantAccess && (v as any).index !== 0 && !userUnlocks.has(v.id)) ? (
+
+                          {!accessible ? (
                             <div className="ml-2">
                               <Button
                                 size="sm"
-                                variant={(v as any).locked ? "secondary" : "default"}
-                                disabled={(v as any).locked || unlockingVideo === v.id}
+                                variant={v.locked ? "secondary" : "default"}
+                                disabled={v.locked || unlockingVideo === v.id}
                                 onClick={(e) => {
                                   e.stopPropagation();
                                   handleUnlock(v);
                                 }}
                               >
-                                {unlockingVideo === v.id ? 'Unlocking...' : (v as any).locked ? 'Locked' : 'Unlock'}
+                                {unlockingVideo === v.id ? 'Unlocking...' : v.locked ? 'Locked' : 'Unlock'}
                               </Button>
                             </div>
                           ) : null}
                         </div>
-                        
+
                         <SegmentedProgressBar
                           segments={playlistSegments[v.id] ?? []}
                           duration={v.durationSec ?? 1}
diff --git a/hooks/useVideo.ts b/hooks/useVideo.ts
index f3f421a..0fdf9cd 100644
--- a/hooks/useVideo.ts
+++ b/hooks/useVideo.ts
@@ -24,7 +24,11 @@ export function useVideo(id?: string) {
 
     fetch(`/api/videos/${id}`)
       .then(async (res) => {
-        if (!res.ok) throw new Error(`HTTP ${res.status}`);
+        if (!res.ok) {
+          // The server explains refusals (e.g. which video to finish first).
+          const body = await res.json().catch(() => null);
+          throw new Error(body?.error ?? `HTTP ${res.status}`);
+        }
         const json = await res.json();
         if (!cancelled) {
           setVideo(json.video ?? null);
@@ -32,7 +36,12 @@ export function useVideo(id?: string) {
         }
       })
       .catch((err) => {
-        if (!cancelled) setError(err);
+        if (!cancelled) {
+          // Never leave the previous video on screen under a new address.
+          setVideo(null);
+          setNext(null);
+          setError(err);
+        }
       })
       .finally(() => {
         if (!cancelled) setIsLoading(false);
diff --git a/lib/account-status.ts b/lib/account-status.ts
new file mode 100644
index 0000000..1239fca
--- /dev/null
+++ b/lib/account-status.ts
@@ -0,0 +1,55 @@
+// lib/account-status.ts
+// Whether an account may currently use the platform, checked against the
+// database rather than whatever was true when the user signed in.
+import { prisma } from './prisma';
+import { normalizeEmail } from './normalize-email';
+
+export type AccountStatus = {
+  /** False once a student is removed from, or deactivated on, the allow-list. */
+  allowed: boolean;
+  role: string;
+  /** Comma-separated course codes from the allow-list. */
+  levels: string;
+};
+
+// Sessions are checked on every request, so results are held briefly. A change
+// to a student's access therefore takes effect within this window.
+const TTL_MS = 60_000;
+const MAX_ENTRIES = 5000;
+const cache = new Map<string, { expires: number; status: AccountStatus }>();
+
+export async function getAccountStatus(
+  userId: string,
+  opts: { fresh?: boolean } = {}
+): Promise<AccountStatus> {
+  const hit = cache.get(userId);
+  if (!opts.fresh && hit && hit.expires > Date.now()) return hit.status;
+
+  const user = await prisma.user.findUnique({
+    where: { id: userId },
+    select: { email: true, role: true },
+  });
+
+  let status: AccountStatus = { allowed: false, role: 'user', levels: '' };
+  if (user) {
+    const role = user.role ?? 'user';
+    const email = normalizeEmail(user.email);
+    const student = email
+      ? await prisma.allowedStudent.findFirst({
+          where: { email: { equals: email, mode: 'insensitive' } },
+          select: { levels: true, active: true },
+        })
+      : null;
+
+    const isAdmin = role === 'admin' || role === 'superadmin';
+    status = {
+      allowed: isAdmin || !!student?.active,
+      role,
+      levels: student?.levels ?? '',
+    };
+  }
+
+  if (cache.size >= MAX_ENTRIES) cache.clear();
+  cache.set(userId, { expires: Date.now() + TTL_MS, status });
+  return status;
+}
diff --git a/lib/auth-options.ts b/lib/auth-options.ts
index 6e0f54d..e31b318 100644
--- a/lib/auth-options.ts
+++ b/lib/auth-options.ts
@@ -4,6 +4,7 @@ import GoogleProvider from "next-auth/providers/google";
 import { PrismaAdapter } from "@next-auth/prisma-adapter";
 import { prisma } from "./prisma";
 import { normalizeEmail } from "./normalize-email";
+import { getAccountStatus } from "./account-status";
 
 export const authOptions: NextAuthOptions = {
   adapter: PrismaAdapter(prisma),
@@ -125,48 +126,30 @@ export const authOptions: NextAuthOptions = {
 
     /**
      * JWT CALLBACK
-     * Token is the source of truth for session data.
-     * Role must ALWAYS come from the database.
+     * Runs on sign-in and again every time the session is read. Role, course
+     * levels and whether the account is still allowed always come from the
+     * database, so removing a student takes effect without waiting for their
+     * 30-day session to expire.
      */
     async jwt({ token, user }) {
-      // On first login
-      if (user?.id) {
-        const dbUser = await prisma.user.findUnique({
-          where: { id: user.id },
-          select: {
-            id: true,
-            role: true,
-            email: true,
-          },
-        });
+      if (user?.id) token.id = user.id;
 
-        if (dbUser) {
-          token.id = dbUser.id;
-          token.role = dbUser.role ?? "user";
-
-          // Attach student levels if applicable
-          let levels = "";
-          const normalizedDbEmail = normalizeEmail(dbUser.email);
-
-          if (normalizedDbEmail) {
-            const allowedStudent = await prisma.allowedStudent.findFirst({
-              where: {
-                email: {
-                  equals: normalizedDbEmail,
-                  mode: "insensitive",
-                },
-              },
-              select: { levels: true },
-            });
-
-            levels = allowedStudent?.levels ?? "";
-          }
-
-          token.levels = levels;
+      const userId = (token.id ?? token.sub) as string | undefined;
+      if (userId) {
+        try {
+          // A fresh sign-in always re-reads the database.
+          const status = await getAccountStatus(userId, { fresh: !!user });
+          token.id = userId;
+          token.role = status.role;
+          token.levels = status.levels;
+          token.disabled = !status.allowed;
+        } catch (err) {
+          // Database hiccup: keep what the token already says rather than
+          // signing everyone out.
+          console.error('[auth] account status check failed', err);
         }
       }
 
-      // Safety fallback
       token.role = token.role ?? "user";
 
       return token;
@@ -174,9 +157,14 @@ export const authOptions: NextAuthOptions = {
 
     /**
      * SESSION CALLBACK
-     * Only exposes data already trusted in the token.
+     * Only exposes data already trusted in the token. A disabled account gets
+     * a session with no user, which every route treats as signed out.
      */
     async session({ session, token }) {
+      if (token.disabled) {
+        return { expires: session.expires };
+      }
+
       if (session.user) {
         (session.user as any).id = token.id ?? null;
         (session.user as any).role = token.role ?? "user";
diff --git a/lib/video-access.ts b/lib/video-access.ts
new file mode 100644
index 0000000..05f9e14
--- /dev/null
+++ b/lib/video-access.ts
@@ -0,0 +1,62 @@
+// lib/video-access.ts
+// Who is asking, and may they watch this video? Shared by the video API and
+// the routes that serve the media files themselves.
+import { getServerSession } from 'next-auth';
+import { authOptions } from './auth-options';
+import { prisma } from './prisma';
+import { checkUnlockEligibility } from './watch-progress';
+
+export type Viewer = { id: string; isAdmin: boolean };
+
+/** The signed-in user, or null. Does not touch the database. */
+export async function getViewer(): Promise<Viewer | null> {
+  const session = await getServerSession(authOptions);
+  const user = session?.user as { id?: string; email?: string; role?: string } | undefined;
+  if (!user?.email || !user.id) return null;
+  return { id: user.id, isAdmin: user.role === 'admin' || user.role === 'superadmin' };
+}
+
+export type AccessResult = { ok: true } | { ok: false; status: number; error: string };
+
+// A playing video requests a media chunk every few seconds, so a positive
+// answer is remembered briefly. Refusals are never cached: an unlock applies
+// straight away, while a newly applied lock can take up to this long to bite.
+const TTL_MS = 60_000;
+const MAX_ENTRIES = 20_000;
+const allowed = new Map<string, number>();
+
+/**
+ * Admins may watch anything. Students need to be enrolled, and the video must
+ * be open to them: first in its playlist, instant access, already unlocked, or
+ * earned by completing the video before it.
+ */
+export async function checkVideoAccess(viewer: Viewer, videoId: string): Promise<AccessResult> {
+  if (viewer.isAdmin) return { ok: true };
+
+  const key = `${viewer.id}:${videoId}`;
+  const expires = allowed.get(key);
+  if (expires && expires > Date.now()) return { ok: true };
+
+  const check = await checkUnlockEligibility(viewer.id, videoId);
+  const where = { userId_videoId: { userId: viewer.id, videoId } };
+
+  if (check.ok) {
+    if (!check.free) {
+      // Earned but never recorded (e.g. completed before unlocks were tracked).
+      await prisma.videoUnlock.upsert({
+        where,
+        update: {},
+        create: { userId: viewer.id, videoId },
+      });
+    }
+  } else {
+    const unlocked =
+      check.reason === 'not_earned' &&
+      (await prisma.videoUnlock.findUnique({ where, select: { id: true } }));
+    if (!unlocked) return { ok: false, status: check.status, error: check.error };
+  }
+
+  if (allowed.size >= MAX_ENTRIES) allowed.clear();
+  allowed.set(key, Date.now() + TTL_MS);
+  return { ok: true };
+}
diff --git a/lib/watch-progress.ts b/lib/watch-progress.ts
new file mode 100644
index 0000000..59aae30
--- /dev/null
+++ b/lib/watch-progress.ts
@@ -0,0 +1,224 @@
+// lib/watch-progress.ts
+// Single source of truth for watch-progress maths and the sequential unlock rule.
+import { prisma } from './prisma';
+
+/** A video counts as completed (and unlocks the next one) at this percentage. */
+export const UNLOCK_THRESHOLD_PERCENT = 80;
+
+export type SecRange = { startSec: number; endSec: number };
+
+/**
+ * Merges overlapping or touching ranges. Returns a new array sorted by start.
+ * When ranges carry a `watchedAt`, the merged range keeps the most recent one.
+ */
+export function mergeRanges<T extends SecRange & { watchedAt?: Date }>(
+  ranges: T[]
+): Array<SecRange & { watchedAt?: Date }> {
+  const sorted = ranges
+    .filter((r) => r.endSec > r.startSec)
+    .sort((a, b) => a.startSec - b.startSec);
+
+  const merged: Array<SecRange & { watchedAt?: Date }> = [];
+  for (const r of sorted) {
+    const last = merged[merged.length - 1];
+    if (last && r.startSec <= last.endSec) {
+      last.endSec = Math.max(last.endSec, r.endSec);
+      if (r.watchedAt && (!last.watchedAt || r.watchedAt > last.watchedAt)) {
+        last.watchedAt = r.watchedAt;
+      }
+    } else {
+      merged.push({ startSec: r.startSec, endSec: r.endSec, watchedAt: r.watchedAt });
+    }
+  }
+  return merged;
+}
+
+export function totalSeconds(ranges: SecRange[]): number {
+  return ranges.reduce((sum, r) => sum + Math.max(0, r.endSec - r.startSec), 0);
+}
+
+/**
+ * Validates client-reported [start, end] pairs and snaps them to whole seconds
+ * inside the video. Anything malformed is dropped.
+ */
+export function sanitizeRanges(input: unknown, durationSec: number): SecRange[] {
+  if (!Array.isArray(input)) return [];
+  const max = Math.ceil(durationSec);
+  const out: SecRange[] = [];
+  for (const item of input.slice(0, 500)) {
+    if (!Array.isArray(item) || item.length < 2) continue;
+    const s = Number(item[0]);
+    const e = Number(item[1]);
+    if (!Number.isFinite(s) || !Number.isFinite(e) || e <= s) continue;
+    const startSec = Math.min(max, Math.max(0, Math.floor(s)));
+    const endSec = Math.min(max, Math.max(0, Math.ceil(e)));
+    if (endSec > startSec) out.push({ startSec, endSec });
+  }
+  return out;
+}
+
+type RestrictableVideo = {
+  videoCourses?: Array<{ courseId: string; exclusive: boolean }>;
+};
+
+/** Drops videos that are exclusive to courses the user is not enrolled in. */
+export function filterAccessibleVideos<T extends RestrictableVideo>(
+  videos: T[],
+  userCourseIds: string[]
+): T[] {
+  return videos.filter((video) => {
+    const restricted = (video.videoCourses ?? [])
+      .filter((a) => a.exclusive)
+      .map((a) => a.courseId);
+    if (restricted.length === 0) return true;
+    return restricted.some((courseId) => userCourseIds.includes(courseId));
+  });
+}
+
+type PlaylistVideo = {
+  id: string;
+  title: string;
+  index: number;
+  locked: boolean;
+  instantAccess: boolean;
+};
+
+/**
+ * The playlist's videos, in order, as this user sees them. Returns null when
+ * the user is not enrolled in any course the playlist belongs to.
+ */
+export async function getPlaylistVideosForUser(
+  userId: string,
+  playlistId: string
+): Promise<PlaylistVideo[] | null> {
+  const [enrollments, playlist] = await Promise.all([
+    prisma.enrollment.findMany({ where: { userId }, select: { courseId: true } }),
+    prisma.playlist.findUnique({
+      where: { id: playlistId },
+      select: {
+        courseId: true,
+        courses: { select: { courseId: true } },
+        videos: {
+          orderBy: { index: 'asc' },
+          select: {
+            id: true,
+            title: true,
+            index: true,
+            locked: true,
+            instantAccess: true,
+            videoCourses: { select: { courseId: true, exclusive: true } },
+          },
+        },
+      },
+    }),
+  ]);
+  if (!playlist) return null;
+
+  const userCourseIds = enrollments.map((e) => e.courseId);
+  const playlistCourseIds = [playlist.courseId, ...playlist.courses.map((c) => c.courseId)];
+  if (!playlistCourseIds.some((id) => userCourseIds.includes(id))) return null;
+
+  return filterAccessibleVideos(playlist.videos, userCourseIds).map(
+    ({ id, title, index, locked, instantAccess }) => ({ id, title, index, locked, instantAccess })
+  );
+}
+
+/**
+ * Called when a user completes `video`: unlocks the next video in the playlist
+ * that needs unlocking (skipping lecturer-locked and instant-access videos).
+ */
+export async function unlockNextVideo(
+  userId: string,
+  video: { id: string; playlistId: string }
+): Promise<{ id: string; title: string } | null> {
+  const videos = await getPlaylistVideosForUser(userId, video.playlistId);
+  if (!videos) return null;
+  const pos = videos.findIndex((v) => v.id === video.id);
+  if (pos === -1) return null;
+
+  const next = videos.slice(pos + 1).find((v) => !v.locked && !v.instantAccess);
+  if (!next) return null;
+
+  await prisma.videoUnlock.upsert({
+    where: { userId_videoId: { userId, videoId: next.id } },
+    update: {},
+    create: { userId, videoId: next.id },
+  });
+  return { id: next.id, title: next.title };
+}
+
+export type UnlockCheck =
+  | { ok: true; free: boolean }
+  | {
+      ok: false;
+      status: number;
+      reason: 'not_found' | 'locked' | 'no_access' | 'not_earned';
+      error: string;
+    };
+
+/**
+ * Decides whether a user may unlock a video by hand. Mirrors unlockNextVideo:
+ * a video opens once any watchable video between it and the previous
+ * unlock-gated video (inclusive) has been completed.
+ * `free` means the video never needed an unlock record.
+ */
+export async function checkUnlockEligibility(
+  userId: string,
+  videoId: string
+): Promise<UnlockCheck> {
+  const video = await prisma.video.findUnique({
+    where: { id: videoId },
+    select: { id: true, playlistId: true, locked: true },
+  });
+  if (!video) return { ok: false, status: 404, reason: 'not_found', error: 'Video not found' };
+  if (video.locked) {
+    return {
+      ok: false,
+      status: 403,
+      reason: 'locked',
+      error: 'This video is locked by your lecturer.',
+    };
+  }
+
+  const videos = await getPlaylistVideosForUser(userId, video.playlistId);
+  const pos = videos ? videos.findIndex((v) => v.id === videoId) : -1;
+  if (!videos || pos === -1) {
+    return {
+      ok: false,
+      status: 403,
+      reason: 'no_access',
+      error: 'You do not have access to this video.',
+    };
+  }
+
+  if (videos[pos].instantAccess || pos === 0) return { ok: true, free: true };
+
+  // Walk back to the previous video that itself needs unlocking.
+  let gateStart = 0;
+  for (let i = pos - 1; i >= 0; i--) {
+    if (!videos[i].locked && !videos[i].instantAccess) {
+      gateStart = i;
+      break;
+    }
+  }
+  const gate = videos.slice(gateStart, pos).filter((v) => !v.locked);
+  if (gate.length === 0) return { ok: true, free: true };
+
+  const progress = await prisma.videoProgress.findMany({
+    where: { userId, videoId: { in: gate.map((v) => v.id) } },
+    select: { videoId: true, percent: true, completed: true },
+  });
+  const done = progress.some(
+    (p) => p.completed || p.percent >= UNLOCK_THRESHOLD_PERCENT
+  );
+  if (done) return { ok: true, free: false };
+
+  const previous = gate[gate.length - 1];
+  const watched = progress.find((p) => p.videoId === previous.id)?.percent ?? 0;
+  return {
+    ok: false,
+    status: 403,
+    reason: 'not_earned',
+    error: `Watch at least ${UNLOCK_THRESHOLD_PERCENT}% of "${previous.title}" to unlock this video (you are at ${watched}%).`,
+  };
+}