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/services/upload.go

@@ -121,18 +121,20 @@ type Box struct {
 }
 
 type File struct {
-	ID                 string    `json:"id"`
-	Name               string    `json:"name"`
-	StoredName         string    `json:"storedName"`
-	Size               int64     `json:"size"`
-	ContentType        string    `json:"contentType"`
-	PreviewKind        string    `json:"previewKind"`
-	Thumbnail          string    `json:"thumbnail,omitempty"`
-	ObjectKey          string    `json:"objectKey,omitempty"`
-	ThumbnailObjectKey string    `json:"thumbnailObjectKey,omitempty"`
-	Processing         bool      `json:"processing,omitempty"`
-	ProcessingError    string    `json:"processingError,omitempty"`
-	UploadedAt         time.Time `json:"uploadedAt"`
+	ID                      string    `json:"id"`
+	Name                    string    `json:"name"`
+	StoredName              string    `json:"storedName"`
+	Size                    int64     `json:"size"`
+	ContentType             string    `json:"contentType"`
+	PreviewKind             string    `json:"previewKind"`
+	Thumbnail               string    `json:"thumbnail,omitempty"`
+	SceneThumbnail          string    `json:"sceneThumbnail,omitempty"`
+	ObjectKey               string    `json:"objectKey,omitempty"`
+	ThumbnailObjectKey      string    `json:"thumbnailObjectKey,omitempty"`
+	SceneThumbnailObjectKey string    `json:"sceneThumbnailObjectKey,omitempty"`
+	Processing              bool      `json:"processing,omitempty"`
+	ProcessingError         string    `json:"processingError,omitempty"`
+	UploadedAt              time.Time `json:"uploadedAt"`
 }
 
 type UploadResult struct {
@@ -731,6 +733,9 @@ func (s *UploadService) RemoveFileFromBox(boxID, fileID string) (bool, error) {
 		if key := s.ThumbnailObjectKey(box, file); key != "" {
 			_ = backend.Delete(context.Background(), key)
 		}
+		if key := s.SceneThumbnailObjectKey(box, file); key != "" {
+			_ = backend.Delete(context.Background(), key)
+		}
 	}
 
 	box.Files = append(box.Files[:index], box.Files[index+1:]...)
@@ -818,6 +823,16 @@ func (s *UploadService) ThumbnailObjectKey(box Box, file File) string {
 	return boxObjectKey(box.ID, file.Thumbnail)
 }
 
+func (s *UploadService) SceneThumbnailObjectKey(box Box, file File) string {
+	if file.SceneThumbnailObjectKey != "" {
+		return file.SceneThumbnailObjectKey
+	}
+	if file.SceneThumbnail == "" {
+		return ""
+	}
+	return boxObjectKey(box.ID, file.SceneThumbnail)
+}
+
 func (s *UploadService) OpenFileObject(ctx context.Context, box Box, file File) (StorageObject, error) {
 	if file.Processing {
 		return StorageObject{}, fmt.Errorf("file is still processing")
@@ -841,6 +856,18 @@ func (s *UploadService) OpenThumbnailObject(ctx context.Context, box Box, file F
 	return backend.Get(ctx, key)
 }
 
+func (s *UploadService) OpenSceneThumbnailObject(ctx context.Context, box Box, file File) (StorageObject, error) {
+	key := s.SceneThumbnailObjectKey(box, file)
+	if key == "" {
+		return StorageObject{}, os.ErrNotExist
+	}
+	backend, err := s.storage.Backend(s.BoxStorageBackendID(box))
+	if err != nil {
+		return StorageObject{}, err
+	}
+	return backend.Get(ctx, key)
+}
+
 func (s *UploadService) PutThumbnailObject(ctx context.Context, box Box, name string, body io.Reader, size int64, contentType string) (string, error) {
 	backend, err := s.storage.Backend(s.BoxStorageBackendID(box))
 	if err != nil {