feat(backend): add video scene preview generation and endpoint

- Register a new route `GET /d/{boxID}/scene/{fileID}` to serve video scene previews.
- Implement the `VideoScenesPreview` handler to serve existing previews or generate them on-demand.
- Add helper functions to analyze video frames (e.g., luma calculation to filter out dark frames) and render the final scene thumbnail.
- Update the `fileView` struct to include scene URL and status fields.

backend/libs/handlers/app.go

@@ -134,6 +134,7 @@ func (a *App) RegisterRoutes(mux *http.ServeMux) {
 	mux.HandleFunc("GET /d/{boxID}/f/{fileID}/download", a.DownloadFileContent)
 	mux.HandleFunc("GET /d/{boxID}/f/{fileID}/og-image.jpg", a.FileOGImage)
 	mux.HandleFunc("GET /d/{boxID}/thumb/{fileID}", a.Thumbnail)
+	mux.HandleFunc("GET /d/{boxID}/scene/{fileID}", a.VideoScenesPreview)
 	mux.HandleFunc("GET /d/{boxID}/og-image.jpg", a.BoxOGImage)
 	mux.HandleFunc("GET /robots.txt", a.RobotsTxt)
 	mux.HandleFunc("GET /sitemap.xml", a.SitemapXML)