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_key_utils.py

@@ -3,7 +3,7 @@ Unit tests for key generation utilities.
 """
 
 import pytest
-from src.core.utils.key_utils import (
+from src.server.utils.key_utils import (
     generate_key_from_folder,
     normalize_key,
     is_valid_key,