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/database/__init__.py

@@ -48,6 +48,7 @@ from src.server.database.service import (
     EpisodeService,
     UserSessionService,
 )
+from src.server.database.SerieList import SerieList
 from src.server.database.system_settings_service import SystemSettingsService
 
 __all__ = [
@@ -79,4 +80,6 @@ __all__ = [
     "DownloadQueueService",
     "SystemSettingsService",
     "UserSessionService",
+    # SerieList
+    "SerieList",
 ]