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>

tests/unit/test_initialization_service.py

@@ -160,7 +160,7 @@ class TestSyncAnimeFolders:
     @pytest.mark.asyncio
     async def test_sync_anime_folders_without_progress(self):
         """Test syncing anime folders without progress service."""
-        with patch('src.server.services.initialization_service.sync_series_from_data_files', 
+        with patch('src.server.services.initialization_service.sync_legacy_series_to_db', 
                    new_callable=AsyncMock, return_value=42) as mock_sync:
             result = await _sync_anime_folders()
         
@@ -172,7 +172,7 @@ class TestSyncAnimeFolders:
         """Test syncing anime folders with progress updates."""
         mock_progress = AsyncMock()
         
-        with patch('src.server.services.initialization_service.sync_series_from_data_files', 
+        with patch('src.server.services.initialization_service.sync_legacy_series_to_db', 
                    new_callable=AsyncMock, return_value=10) as mock_sync:
             result = await _sync_anime_folders(progress_service=mock_progress)