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/upload/finalize/route.ts

@@ -4,8 +4,7 @@ import fs from "fs";
 import path from "path";
 import { spawn } from "child_process";
 import { prisma } from "@/lib/prisma";
-import { getServerSession } from "next-auth";
-import { authOptions } from "@/lib/auth-options";
+import { requireAdmin } from "@/lib/admin-check";
 
 const UPLOADS_DIR = process.env.UPLOADS_DIR || "/uploads";
 
@@ -56,23 +55,11 @@ async function getVideoDurationFromFile(filePath: string): Promise<number | null
   });
 }
 
-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" };
-  return session;
-}
-
 export async function POST(req: Request) {
   try {
     console.log('[Upload/Finalize] Request received');
     
-    await checkAdmin();
+    await requireAdmin();
     
     const body = await req.json();
     const { videoId } = body;