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/api/config.py

@@ -284,9 +284,9 @@ async def update_directory(
         try:
             import structlog
 
-            from src.server.services.anime_service import sync_series_from_data_files
+            from src.server.services.anime_service import sync_legacy_series_to_db
             logger = structlog.get_logger(__name__)
-            sync_count = await sync_series_from_data_files(directory, logger)
+            sync_count = await sync_legacy_series_to_db(directory, logger)
             logger.info(
                 "Directory updated: synced series from data files",
                 directory=directory,