Introduce comprehensive tests for the Event model, covering creation, updates, deletions, relationships, defaults, and constraints. Added the `mock_user` fixture to streamline user-related test setups and ensure proper linkage between users and events. Improves test coverage and ensures model behavior consistency.
Introduce a `db_session` pytest fixture to provide a fresh SQLite in-memory database for each test function. This ensures isolated and consistent database state across tests by setting up and tearing down the database automatically.
