refactor: restructure core→server, split large entity files into database module

- Move src/core/ → src/server/
- Split SerieList.py (531 lines) and series.py (414 lines) into src/server/database/
- Add database/models.py for SQLAlchemy models
- Update all test imports to reflect new structure
- Remove deprecated test files (test_serie_class.py, test_serie_folder_with_year.py)

tests/unit/test_concurrent_scans.py

@@ -535,7 +535,7 @@ class TestAnimeServiceScanLock:
     @pytest.mark.asyncio
     async def test_anime_service_ignores_concurrent_rescan_requests(self):
         """Test that AnimeService ignores concurrent rescan requests."""
-        from src.core.SeriesApp import SeriesApp
+        from src.server.SeriesApp import SeriesApp
         from src.server.services.anime_service import AnimeService
 
         # Mock database