Add function to relocate theme files in storage

Introduce `_relocate_theme_file` to handle moving files to theme-specific directories in the storage system. This ensures better organization of uploaded files by associating them with a theme ID and file type, improving maintainability and structure.

backend/app/core/storage.py

@@ -11,7 +11,7 @@ from app.core.config import settings
 
 class StorageProvider(ABC):
     """Base abstract class for storage providers."""
-
+    upload_folder: Path
     @abstractmethod
     async def save_file(self, file: UploadFile, destination: str) -> str:
         """Save a file to storage and return the relative path."""