fix: prevent duplicate series when same anime key exists in different folder

- Add check for existing series by key in SetupService.run to skip duplicates
- Fix Path construction in initialization_service.py cleanup function
- Update unit tests to mock get_by_key and get_series_app

src/server/services/initialization_service.py

@@ -165,7 +165,7 @@ async def _cleanup_legacy_key_files() -> int:
             db_folders: set[str] = {series.folder for series in all_series if series.folder}
 
             for folder_name in db_folders:
-                folder_path = settings.anime_directory / folder_name
+                folder_path = Path(settings.anime_directory) / folder_name
                 key_file = folder_path / "key"
 
                 if not key_file.exists():