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/meta.go

@@ -23,6 +23,7 @@ Disallow: /account/
 Disallow: /d/*/f/*/download
 Disallow: /d/*/zip
 Disallow: /d/*/thumb/
+Disallow: /d/*/scene/
 Disallow: /d/*/og-image.jpg
 Disallow: /d/*/unlock
 Disallow: /d/*/manage/