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/static/css/30-download.css

@@ -260,6 +260,11 @@
   height: clamp(18rem, 64vh, 38rem);
 }
 
+.video-scenes-preview {
+  object-fit: contain;
+  background: color-mix(in srgb, var(--background) 88%, black 12%);
+}
+
 .native-audio-preview {
   align-self: center;
   width: min(42rem, calc(100% - 2rem));