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_callbacks.py

@@ -7,7 +7,7 @@ functionality.
 
 import unittest
 
-from src.core.interfaces.callbacks import (
+from src.server.interfaces.callbacks import (
     CallbackManager,
     CompletionCallback,
     CompletionContext,