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/conftest.py

@@ -131,7 +131,7 @@ def mock_series_app_download(monkeypatch):
     """
     # Mock the loader download method
     try:
-        from src.core.SeriesApp import SeriesApp
+        from src.server.SeriesApp import SeriesApp
 
         # Patch the loader.download method for all SeriesApp instances
         original_init = SeriesApp.__init__