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/thumbnails/[...path]/route.ts

@@ -27,7 +27,7 @@ export async function GET(
     const requestedPath = path.join(THUMBNAILS_DIR, ...pathSegments);
     
     // Security: prevent directory traversal attacks
-    if (!requestedPath.startsWith(THUMBNAILS_DIR)) {
+    if (!requestedPath.startsWith(THUMBNAILS_DIR + path.sep)) {
       return NextResponse.json(
         { error: 'Invalid request' },
         { status: 403 }
@@ -93,7 +93,7 @@ export async function HEAD(
     const requestedPath = path.join(THUMBNAILS_DIR, ...pathSegments);
     
     // Security: prevent directory traversal attacks
-    if (!requestedPath.startsWith(THUMBNAILS_DIR)) {
+    if (!requestedPath.startsWith(THUMBNAILS_DIR + path.sep)) {
       return new NextResponse(null, { status: 403 });
     }