Add session management API, cleanup service, and session-specific tests

- Introduced session management endpoints to list, revoke, and cleanup sessions per user.
- Added cron-based job for periodic cleanup of expired sessions.
- Implemented `CRUDSession` for session-specific database operations.
- Integrated session cleanup startup and shutdown events in the application lifecycle.
- Enhanced CORS configuration to include `X-Device-Id` for session tracking.
- Added comprehensive integration tests for multi-device login, per-device logout, session listing, and cleanup logic.

backend/tests/conftest.py

@@ -1,10 +1,15 @@
 # tests/conftest.py
+import os
 import uuid
 from datetime import datetime, timezone
 
 import pytest
 from fastapi.testclient import TestClient
 
+# Set IS_TEST environment variable BEFORE importing app
+# This prevents the scheduler from starting during tests
+os.environ["IS_TEST"] = "True"
+
 from app.main import app
 from app.core.database import get_db
 from app.models.user import User