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>

package.json

@@ -45,11 +45,11 @@
     "formidable": "^3.5.4",
     "hls.js": "^1.6.15",
     "lucide-react": "^0.554.0",
-    "next": "16.0.3",
-    "next-auth": "^4.24.13",
+    "next": "16.3.5",
+    "next-auth": "^4.24.15",
     "next-themes": "^0.4.6",
-    "react": "19.2.0",
-    "react-dom": "19.2.0",
+    "react": "19.3.0",
+    "react-dom": "19.3.0",
     "recharts": "2.15.4",
     "sonner": "^2.0.7",
     "swr": "^2.3.6",
@@ -61,13 +61,13 @@
   "devDependencies": {
     "@tailwindcss/postcss": "^4.1.17",
     "@types/formidable": "^3.4.6",
-    "@types/unzipper": "^0.10.10",
     "@types/node": "^20.19.25",
     "@types/react": "^19",
     "@types/react-dom": "^19",
+    "@types/unzipper": "^0.10.10",
     "autoprefixer": "^10.4.22",
     "eslint": "^9",
-    "eslint-config-next": "16.0.3",
+    "eslint-config-next": "16.3.5",
     "postcss": "^8.5.6",
     "prisma": "^6.19.0",
     "tailwindcss": "^4.1.17",