- flask-socketio + threading mode (no eventlet monkey-patching) - preview_queue in pipeline: downscale to 640p, JPEG encode, bounded queue - encoder thread per job reads queue → SocketIO broadcast to room - Frontend: SocketIO client replaces HTTP polling, handles preview_frame event - Configurable: preview_every_n, preview_max_dim, preview_jpeg_quality - 62/62 tests pass