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/anime_service.py

@@ -1554,19 +1554,17 @@ def get_anime_service(series_app: SeriesApp) -> AnimeService:
     return AnimeService(series_app)
 
 
-async def sync_series_from_data_files(
+async def sync_legacy_series_to_db(
     anime_directory: str,
     log_instance=None  # pylint: disable=unused-argument
 ) -> int:
     """
-    Sync series from data files to the database.
+    One-time legacy sync: import any series from 'data' files
+    not already in the database.
 
-    Scans the anime directory for data files and adds any new series
-    to the database. Existing series are skipped (no duplicates).
-
-    This function is typically called during application startup to ensure
-    series metadata stored in filesystem data files is available in the
-    database.
+    Deprecated: Series are now loaded directly from the database.
+    This function remains for backwards compatibility with legacy
+    file-based data during migration.
 
     Args:
         anime_directory: Path to the anime directory with data files
@@ -1578,6 +1576,11 @@ async def sync_series_from_data_files(
     """
     # Always use structlog for structured logging with keyword arguments
     log = structlog.get_logger(__name__)
+        
+    log.warning(
+        "sync_legacy_series_to_db is deprecated. "
+        "Series are now loaded directly from database."
+    )
 
     try:
         from src.server.database.connection import get_db_session