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_anime_service.py

@@ -16,7 +16,7 @@ import pytest
 from src.server.services.anime_service import (
     AnimeService,
     AnimeServiceError,
-    sync_series_from_data_files,
+    sync_legacy_series_to_db,
 )
 from src.server.services.progress_service import ProgressService
 
@@ -1303,7 +1303,7 @@ class TestGetNFOStatisticsSelfManaged:
 
 
 class TestSyncSeriesFromDataFiles:
-    """Test module-level sync_series_from_data_files function."""
+    """Test module-level sync_legacy_series_to_db function."""
 
     @pytest.mark.asyncio
     async def test_sync_adds_new_series(self, tmp_path):
@@ -1343,7 +1343,7 @@ class TestSyncSeriesFromDataFiles:
             ]
             MockApp.return_value = mock_app_instance
 
-            count = await sync_series_from_data_files(str(tmp_path))
+            count = await sync_legacy_series_to_db(str(tmp_path))
 
         assert count == 1
         mock_create.assert_called_once()
@@ -1382,7 +1382,7 @@ class TestSyncSeriesFromDataFiles:
             ]
             MockApp.return_value = mock_app_instance
 
-            count = await sync_series_from_data_files(str(tmp_path))
+            count = await sync_legacy_series_to_db(str(tmp_path))
 
         assert count == 0
         mock_create.assert_not_called()
@@ -1397,7 +1397,7 @@ class TestSyncSeriesFromDataFiles:
             mock_app_instance.get_all_series_from_data_files.return_value = []
             MockApp.return_value = mock_app_instance
 
-            count = await sync_series_from_data_files(str(tmp_path))
+            count = await sync_legacy_series_to_db(str(tmp_path))
 
         assert count == 0
 
@@ -1436,7 +1436,7 @@ class TestSyncSeriesFromDataFiles:
             ]
             MockApp.return_value = mock_app_instance
 
-            count = await sync_series_from_data_files(str(tmp_path))
+            count = await sync_legacy_series_to_db(str(tmp_path))
 
         assert count == 1
         # The name should have been set to folder