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/services/scheduler/folder_scan_service.py

@@ -14,7 +14,7 @@ import structlog
 from lxml import etree
 
 from src.config.settings import settings as _settings
-from src.core.utils.image_downloader import ImageDownloader
+from src.server.utils.image_downloader import ImageDownloader
 
 logger = structlog.get_logger(__name__)