Fix refresh token expiry check for naive datetime comparison

[Mng-dev-ai]

Summary

• Fix RefreshToken.is_expired to handle naive (timezone-unaware) expires_at datetimes by assuming UTC before comparing with datetime.now(timezone.utc)
• Prevents TypeError when the database returns a naive datetime

Test plan

• Verify refresh token expiry works correctly with timezone-aware datetimes
• Verify refresh token expiry works correctly with naive datetimes from the database