Patch React2Shell RCE and harden auth, uploads and containers

- Upgrade next 16.0.3 -> 16.3.5 and react/react-dom 19.2.0 -> 19.3.0
  (CVE-2025-55182 / CVE-2025-66478 unauthenticated RCE), next-auth -> 4.24.15
- Require admin role on /api/users, /api/courses and /api/enrollments,
  which were unauthenticated
- Replace ALLOWED_ADMINS checks that allowed any signed-in user when the
  variable was empty with the role-based requireAdmin
- Sanitize upload filenames, restrict thumbnail extensions, store edited
  thumbnails in UPLOADS_DIR, tighten traversal checks in file routes
- Dockerfile: Node 22, pinned pnpm, non-root user, critical audit gate
- Compose: read-only rootfs, dropped caps, no-new-privileges, Postgres
  password from env
- Exclude .env from Docker build context; add security headers
- Deploy with git reset --hard instead of git pull
- Remove stale package-lock.json

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/api/admin/create-playlist/route.ts

@@ -1,19 +1,11 @@
 // app/api/admin/create-playlist/route.ts
 import { NextResponse } from 'next/server';
 import { prisma } from '@/lib/prisma';
-import { getServerSession } from 'next-auth';
-import { authOptions } from '@/lib/auth-options';
-
-async function checkAdmin() {
-  const session = await getServerSession(authOptions);
-  if (!session?.user?.email) throw { status: 401, message: 'Unauthorized' };
-  const allowed = (process.env.ALLOWED_ADMINS || '').split(',').map(s=>s.trim()).filter(Boolean);
-  if (allowed.length && !allowed.includes(session.user.email)) throw { status: 403, message: 'Forbidden' };
-}
+import { requireAdmin } from '@/lib/admin-check';
 
 export async function POST(req: Request) {
   try {
-    await checkAdmin();
+    await requireAdmin();
     const { title, courseId, additionalCourseIds } = await req.json();
     if (!title || !courseId) return NextResponse.json({ error: 'missing fields' }, { status: 400 });