Fix auth datetime comparison and bcrypt dependency

- Use timezone-aware datetimes (datetime.now(timezone.utc)) for session expiry comparison
- Add explicit bcrypt==4.0.1 dependency for passlib bcrypt backend

backend/requirements.txt

@@ -9,6 +9,7 @@ pydantic==2.5.3
 pydantic-settings==2.1.0
 python-jose[cryptography]==3.3.0
 passlib[bcrypt]==1.7.4
+bcrypt==4.0.1
 
 # Test dependencies
 pytest>=7.4.0