feat(backend): handle processing errors and add PWA routes

- Block file downloads and previews with a 424 StatusFailedDependency if file processing failed or the box has issues.
- Register routes for `/service-worker.js` and `/share-target` to support PWA features.
- Update README.md with an AI usage disclosure.

backend/libs/services/resumable.go

@@ -369,19 +369,20 @@ func (s *UploadService) FinalizeProcessingResumableSession(ctx context.Context,
 	}
 	backend, err := s.storage.Backend(box.StorageBackendID)
 	if err != nil {
+		_ = s.markProcessingBoxFailed(box, err)
 		return UploadResult{}, err
 	}
 	for i, incoming := range staged {
 		source, err := incoming.Open()
 		if err != nil {
+			_ = s.markProcessingBoxFailed(box, err)
 			return UploadResult{}, err
 		}
 		file := box.Files[i]
 		if err := s.writeUploadedObject(ctx, backend, file.ObjectKey, source, incoming.Size(), 0, incoming.ContentType()); err != nil {
 			source.Close()
 			_ = backend.Delete(context.Background(), file.ObjectKey)
-			box.Files[i].ProcessingError = err.Error()
-			_ = s.saveBoxRecord(box)
+			_ = s.markProcessingBoxFailed(box, err)
 			return UploadResult{}, err
 		}
 		source.Close()
@@ -406,6 +407,35 @@ func (s *UploadService) FinalizeProcessingResumableSession(ctx context.Context,
 	return s.resultForBox(box, ""), nil
 }
 
+func (s *UploadService) markProcessingBoxFailed(box Box, cause error) error {
+	message := "upload processing failed"
+	if cause != nil && strings.TrimSpace(cause.Error()) != "" {
+		message = cause.Error()
+	}
+	s.logger.Warn("resumable upload box marked failed", "source", "user-upload", "severity", "warn", "code", 4021, "box_id", box.ID, "backend_id", s.BoxStorageBackendID(box), "files", len(box.Files), "error", message)
+	now := time.Now().UTC()
+	box.Trouble = true
+	box.TroubleReason = message
+	for i := range box.Files {
+		if box.Files[i].Processing || box.Files[i].ProcessingError == "" {
+			box.Files[i].Processing = false
+			box.Files[i].ProcessingError = message
+			if box.Files[i].UploadedAt.IsZero() {
+				box.Files[i].UploadedAt = now
+			}
+		}
+	}
+	if err := s.saveBoxRecord(box); err != nil {
+		s.logger.Warn("failed to save failed upload box state", "source", "user-upload", "severity", "warn", "code", 4022, "box_id", box.ID, "backend_id", s.BoxStorageBackendID(box), "error", err.Error())
+		return err
+	}
+	if err := s.writeBoxMetadata(box); err != nil {
+		s.logger.Warn("failed to write failed upload box metadata", "source", "user-upload", "severity", "warn", "code", 4023, "box_id", box.ID, "backend_id", s.BoxStorageBackendID(box), "error", err.Error())
+		return err
+	}
+	return nil
+}
+
 func (s *UploadService) CompleteUploadedResumableSession(ctx context.Context, sessionID string) (UploadResult, ResumableSession, error) {
 	session, err := s.GetResumableSession(sessionID)
 	if err != nil {