Add takeover_enabled, takeover_mode, and takeover_delay_seconds columns
to the Authorization model with a SQLite migration, and include them
in to_dict(). Test: test_takeover_model.py.
- Add POST /api/avatar/{id}/photo endpoint
- Save uploaded photos to UPLOAD_DIR/{avatar_id}/
- Update avatar.photo_url in database
- Import UPLOAD_DIR from knowledge router
stefanfeng