feat(file): conceal file resource existence from other users

- feat: cross-user file access returns not-found instead of
  permission-denied
- fix: repository delete now only affects active rows and returns
  ErrNotFound when no row changes; repeated deletes yield not-found
- feat: parent directory operations (list, upload, create-dir, move)
  conceal ownership of other user's directories
- test: update handler, service, repository, and integration tests to
  assert not-found behavior for cross-user and missing file operations

internal/service/file.go

@@ -304,10 +304,6 @@ func (s *FileService) Update(ctx context.Context, userID, fileID string, newName
 func (s *FileService) Delete(ctx context.Context, userID, fileID string) error {
 	file, err := s.getOwnedFile(ctx, userID, fileID)
 	if err != nil {
-		var ae *model.AppError
-		if errors.As(err, &ae) && errors.Is(ae.Err, model.ErrNotFound) {
-			return nil // idempotent: already deleted
-		}
 		return err
 	}
 
@@ -323,6 +319,9 @@ func (s *FileService) Delete(ctx context.Context, userID, fileID string) error {
 	}
 
 	if err := s.fileRepo.Delete(ctx, fileID); err != nil {
+		if errors.Is(err, model.ErrNotFound) {
+			return model.NewNotFoundError("file not found", model.ErrNotFound)
+		}
 		return model.NewInternalError("delete file record", err)
 	}
 
@@ -378,7 +377,7 @@ func (s *FileService) getOwnedFile(ctx context.Context, userID, fileID string) (
 	}
 
 	if file.UserID != userID {
-		return nil, model.NewForbiddenError("access denied", model.ErrForbidden)
+		return nil, model.NewNotFoundError("file not found", model.ErrNotFound)
 	}
 
 	return file, nil
@@ -395,7 +394,7 @@ func (s *FileService) verifyParent(ctx context.Context, userID, parentID string)
 	}
 
 	if parent.UserID != userID {
-		return model.NewForbiddenError("access denied", model.ErrForbidden)
+		return model.NewNotFoundError("parent directory not found", model.ErrNotFound)
 	}
 
 	if !parent.IsDir {