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_failover.py

@@ -1,7 +1,7 @@
 """Unit tests for provider failover system."""
 import pytest
 
-from src.core.providers.failover import (
+from src.server.providers.failover import (
     ProviderFailover,
     configure_failover,
     get_failover,