feat: add legacy key/data file migration to database

- Add migration_legacy_files_completed flag to SystemSettings model
- Create legacy_file_migration service to migrate series from key/data files
- Integrate legacy migration into initialization_service startup flow
- Add integration tests for legacy file migration
- Update DATABASE.md documentation with migration details
- Fix various test and service issues (nfo_repair, tmdb_client, download_service)
- Add test_database_schema unit tests

tests/unit/test_queue_operations.py

@@ -27,7 +27,9 @@ def _make_episode(season: int = 1, episode: int = 1) -> EpisodeIdentifier:
 
 @pytest.fixture
 def mock_anime_service():
-    return MagicMock(spec=["download_episode"])
+    service = MagicMock(spec=["download_episode"])
+    service._directory = "/tmp/test_anime"
+    return service
 
 
 @pytest.fixture