feat(backend): implement on-demand thumbnail generation

When a thumbnail is requested but not yet available, attempt to generate
it synchronously on-demand instead of immediately falling back to the
placeholder image.

- Export thumbnail generation helpers from the jobs package.
- Update the Thumbnail handler to trigger on-demand generation if the
  thumbnail object is missing.
- Save the updated box metadata with the new thumbnail reference.
- Fall back to the placeholder image only if on-demand generation fails.

backend/libs/jobs/thumbnails.go

@@ -141,6 +141,14 @@ func needsThumbnail(file services.File) bool {
 	return file.PreviewKind == "image" || file.PreviewKind == "video" || isTextThumbnailCandidate(file)
 }
 
+func NeedsThumbnail(file services.File) bool {
+	return needsThumbnail(file)
+}
+
+func GenerateThumbnailForFile(uploadService *services.UploadService, box services.Box, file services.File) (string, error) {
+	return generateThumbnail(uploadService, box, file)
+}
+
 func generateThumbnail(uploadService *services.UploadService, box services.Box, file services.File) (string, error) {
 	thumbnailName := "@thumb@" + file.ID + ".jpg"
 	object, err := uploadService.OpenFileObject(context.Background(), box, file)