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/integration/test_concurrent_operations.py

@@ -60,7 +60,7 @@ class TestCacheConsistency:
 
     def test_provider_cache_key_uniqueness(self):
         """Different inputs produce different cache keys."""
-        from src.core.providers.aniworld_provider import AniworldLoader
+        from src.server.providers.aniworld_provider import AniworldLoader
 
         loader = AniworldLoader.__new__(AniworldLoader)
         loader.cache = {}