feat(download): add dynamic OG metadata and fix thumbnail caching

- 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.

backend/libs/handlers/app.go

@@ -91,6 +91,7 @@ func (a *App) RegisterRoutes(mux *http.ServeMux) {
 	mux.HandleFunc("GET /d/{boxID}/f/{fileID}", a.DownloadFile)
 	mux.HandleFunc("GET /d/{boxID}/f/{fileID}/download", a.DownloadFileContent)
 	mux.HandleFunc("GET /d/{boxID}/thumb/{fileID}", a.Thumbnail)
+	mux.HandleFunc("GET /d/{boxID}/og-image.jpg", a.BoxOGImage)
 	mux.HandleFunc("GET /health", a.Health)
 	mux.HandleFunc("GET /healthz", a.Health)
 	mux.HandleFunc("GET /api/v1/health", a.Health)