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/jobs/thumbnails_test.go

@@ -72,6 +72,52 @@ func TestNeedsThumbnailIncludesCodeTextFiles(t *testing.T) {
 	}
 }
 
+func TestUsableVideoFrameRejectsBlackFrame(t *testing.T) {
+	var dark bytes.Buffer
+	if err := jpeg.Encode(&dark, solidTestImage(color.RGBA{A: 255}), nil); err != nil {
+		t.Fatalf("jpeg.Encode dark returned error: %v", err)
+	}
+	if usableVideoFrame(dark.Bytes()) {
+		t.Fatalf("black video frame should not be usable")
+	}
+
+	var bright bytes.Buffer
+	if err := jpeg.Encode(&bright, solidTestImage(color.RGBA{R: 180, G: 80, B: 40, A: 255}), nil); err != nil {
+		t.Fatalf("jpeg.Encode bright returned error: %v", err)
+	}
+	if !usableVideoFrame(bright.Bytes()) {
+		t.Fatalf("bright video frame should be usable")
+	}
+}
+
+func TestRenderVideoScenesThumbnailReturnsLargeJPEG(t *testing.T) {
+	data := renderVideoScenesThumbnail(
+		services.File{Name: "clip.mp4", ContentType: "video/mp4"},
+		videoInfo{Codec: "h264", Width: 1920, Height: 1080, Duration: 125, FrameRate: "24.00 fps"},
+		[]videoSceneFrame{
+			{Timestamp: "00:00:10", Image: solidTestImage(color.RGBA{R: 140, G: 40, B: 80, A: 255})},
+			{Timestamp: "00:00:35", Image: solidTestImage(color.RGBA{R: 40, G: 120, B: 150, A: 255})},
+		},
+	)
+	img, err := jpeg.Decode(bytes.NewReader(data))
+	if err != nil {
+		t.Fatalf("jpeg.Decode returned error: %v", err)
+	}
+	if img.Bounds().Dx() != 1200 || img.Bounds().Dy() != 630 {
+		t.Fatalf("scene preview size = %dx%d, want 1200x630", img.Bounds().Dx(), img.Bounds().Dy())
+	}
+}
+
+func solidTestImage(c color.Color) image.Image {
+	img := image.NewRGBA(image.Rect(0, 0, 32, 24))
+	for y := 0; y < img.Bounds().Dy(); y++ {
+		for x := 0; x < img.Bounds().Dx(); x++ {
+			img.Set(x, y, c)
+		}
+	}
+	return img
+}
+
 func newThumbnailTestUploadService(t *testing.T) *services.UploadService {
 	t.Helper()
 	service, err := services.NewUploadService(1024*1024, t.TempDir(), "http://example.test", slog.New(slog.NewTextHandler(io.Discard, nil)))