Rename sync_series_from_data_files to sync_legacy_series_to_db

- Rename function to reflect its legacy status
- Add deprecation warning log on execution
- Update all callers (initialization_service, api/config, fastapi_app)
- Update tests to use new name
- Add deprecation notice to DEVELOPMENT.md

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/server/services/initialization_service.py

@@ -6,7 +6,7 @@ from typing import Callable, Optional
 import structlog
 
 from src.config.settings import settings
-from src.server.services.anime_service import sync_series_from_data_files
+from src.server.services.anime_service import sync_legacy_series_to_db
 from src.server.services.legacy_file_migration import migrate_series_from_files_to_db
 
 logger = structlog.get_logger(__name__)
@@ -170,7 +170,7 @@ async def _sync_anime_folders(progress_service=None) -> int:
             metadata={"step_id": "series_sync"}
         )
     
-    sync_count = await sync_series_from_data_files(settings.anime_directory)
+    sync_count = await sync_legacy_series_to_db(settings.anime_directory)
     logger.info("Data file sync complete. Added %d series.", sync_count)
     
     if progress_service: