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_provider_health.py

@@ -4,7 +4,7 @@ from datetime import datetime
 
 import pytest
 
-from src.core.providers.health_monitor import (
+from src.server.providers.health_monitor import (
     ProviderHealthMetrics,
     ProviderHealthMonitor,
     RequestMetric,