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>

lib/admin-check.ts

@@ -1,4 +1,5 @@
 // lib/admin-check.ts
+import { NextResponse } from 'next/server';
 import { getServerSession } from 'next-auth';
 import { authOptions } from './auth-options';
 
@@ -9,3 +10,19 @@ export async function requireAdmin() {
   if (!(role === 'admin' || role === 'superadmin')) throw { status: 403, message: 'Forbidden' };
   return session;
 }
+
+/**
+ * Route-handler variant: returns a JSON error response when the caller is not
+ * an admin, or null when they are.
+ */
+export async function adminGuard(): Promise<NextResponse | null> {
+  try {
+    await requireAdmin();
+    return null;
+  } catch (err: any) {
+    return NextResponse.json(
+      { error: err?.message ?? 'Unauthorized' },
+      { status: err?.status ?? 401 }
+    );
+  }
+}