fix(scanner, browse): fallback to original image when thumbnail generation fails for cover.jpg

When scanning image files (e.g. cover.jpg in audiobook folders),
thumbnailForImage can fail or produce a missing .thumbnails/cover.jpg.
This caused broken thumbnails in the grid.

- scanner.go buildThumbnailPath: after generating an image thumbnail,
  verify the output file exists via fs.Stat. If it does not exist,
  fall back to using the original image path as the thumbnail_path.
- browse.go GetThumbnail: if os.Stat(ThumbnailPath) fails and the
  media type is image, fall back to serving the original AbsPath.

All tests pass.

2 files changed, 29 insertions, 8 deletions

diff --git a/internal/scanner/scanner.go b/internal/scanner/scanner.go
index 30b20d1..34e0ca5 100644
--- a/internal/scanner/scanner.go
+++ b/internal/scanner/scanner.go
@@ -202,7 +202,16 @@ func (s *FSScanner) buildThumbnailPath(ctx context.Context, path, setPath string
 		if ext == ".svg" {
 			return path, nil
 		}
-		return s.thumbnailForImage(ctx, path, setPath)
+		thumbPath, err := s.thumbnailForImage(ctx, path, setPath)
+		if err != nil {
+			return "", err
+		}
+		if thumbPath != "" {
+			if _, statErr := s.fs.Stat(thumbPath); statErr == nil {
+				return thumbPath, nil
+			}
+		}
+		return path, nil
 	}
 	return "", nil
 }
diff --git a/internal/service/browse.go b/internal/service/browse.go
index b420d70..0df399b 100644
--- a/internal/service/browse.go
+++ b/internal/service/browse.go
@@ -183,14 +183,26 @@ func (s *browseService) GetThumbnail(ctx context.Context, mediaID, userID int64)
 		return nil, errors.New("thumbnail not found")
 	}
 	info, err := os.Stat(media.ThumbnailPath)
-	if err != nil {
-		return nil, fmt.Errorf("stat thumbnail: %w", err)
+	if err == nil {
+		return &FileResult{
+			Path:     media.ThumbnailPath,
+			FileName: filepath.Base(media.ThumbnailPath),
+			FileSize: info.Size(),
+		}, nil
 	}
-	return &FileResult{
-		Path:     media.ThumbnailPath,
-		FileName: filepath.Base(media.ThumbnailPath),
-		FileSize: info.Size(),
-	}, nil
+	// If the generated thumbnail is missing, fall back to the original file
+	// for image media so that cover.jpg and similar files still render.
+	if media.Type == model.MediaTypeImage {
+		info, err = os.Stat(media.AbsPath)
+		if err == nil {
+			return &FileResult{
+				Path:     media.AbsPath,
+				FileName: filepath.Base(media.AbsPath),
+				FileSize: info.Size(),
+			}, nil
+		}
+	}
+	return nil, fmt.Errorf("stat thumbnail: %w", err)
 }
 
 func (s *browseService) RegenerateThumbnail(ctx context.Context, mediaID, userID int64) error {