Add storage utilities and tests for file handling and tokens

Introduced new fixtures and tests for storage functionality, including saving files, generating URLs, and token creation/verification. Refactored `get_storage_provider` into a separate dependency module. Enhanced test coverage for improved reliability.

backend/tests/conftest.py

@@ -19,6 +19,7 @@ from app.models import Event, GiftItem, GiftStatus, GiftPriority, GiftCategory,
 from app.models.user import User
 from app.utils.test_utils import setup_test_db, teardown_test_db, setup_async_test_db, teardown_async_test_db
 
+pytest_plugins = ["pytest_asyncio"]
 
 @pytest.fixture(scope="function")
 def db_session():