Add library folder validation service method

- Add HasFolders() method to LibraryService
- Validates library has at least one folder before operations
- Returns clear boolean result
- Follows service layer architecture pattern

Related: TestCollectionsBulkOperations fix

internal/services/library_service.go

@@ -198,3 +198,12 @@ func (s *LibraryService) GetLibraryStats(ctx context.Context, libraryID pgtype.U
 		"media_count": len(mediaItems),
 	}, nil
 }
+
+// HasFolders checks if a library has at least one folder configured
+func (s *LibraryService) HasFolders(ctx context.Context, libraryID pgtype.UUID) (bool, error) {
+	folders, err := s.db.GetLibraryFolders(ctx, libraryID)
+	if err != nil {
+		return false, fmt.Errorf("failed to check library folders: %w", err)
+	}
+	return len(folders) > 0, nil
+}