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)

src/server/core_init_merge.py

@@ -0,0 +1,8 @@
+"""
+Core module for AniWorld application.
+Contains domain entities, interfaces, application services, and exceptions.
+"""
+
+from . import entities, exceptions, interfaces, providers
+
+__all__ = ['entities', 'exceptions', 'interfaces', 'providers']