Add init_db script for async database initialization and extensive tests for session management

- Added `init_db.py` to handle async database initialization with the creation of the first superuser if configured.
- Introduced comprehensive tests for session management APIs, including session listing, revocation, and cleanup.
- Enhanced CRUD session logic with UUID utilities and improved error handling.

backend/app/crud/session.py

@@ -2,6 +2,7 @@
 Async CRUD operations for user sessions using SQLAlchemy 2.0 patterns.
 """
 import logging
+import uuid
 from datetime import datetime, timezone, timedelta
 from typing import List, Optional
 from uuid import UUID