- Register a new route for box Open Graph images (`/d/{boxID}/og-image.jpg`).
- Dynamically set the download page title, description, and OG image URL based on box state (e.g., file count, expiration, password protection).
- Introduce `servePlaceholderThumbnail` to serve fallback thumbnails with `Cache-Control: no-store, must-revalidate`. This ensures the browser requests the real thumbnail once it is generated instead of caching the placeholder.
