Upgrade Next.js to 16.4.6 and show locks on latest videos

- Upgrade next and eslint-config-next from 16.3.5 to 16.4.6. The older
  version has critical advisories, which made the Docker build's
  `pnpm audit --audit-level critical` step fail and blocked deploys.
- /api/videos/latest now reports whether the viewer can open each video;
  the dashboard shows a lock and the reason instead of opening the player.

Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>

package.json

@@ -45,7 +45,7 @@
     "formidable": "^3.5.4",
     "hls.js": "^1.6.15",
     "lucide-react": "^0.554.0",
-    "next": "16.3.5",
+    "next": "16.4.6",
     "next-auth": "^4.24.15",
     "next-themes": "^0.4.6",
     "react": "19.3.0",
@@ -67,7 +67,7 @@
     "@types/unzipper": "^0.10.10",
     "autoprefixer": "^10.4.22",
     "eslint": "^9",
-    "eslint-config-next": "16.3.5",
+    "eslint-config-next": "16.4.6",
     "postcss": "^8.5.6",
     "prisma": "^6.19.0",
     "tailwindcss": "^4.1.17",