feat: 15+ FPS live preview via WebSocket push

- 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

templates/base.html

@@ -35,5 +35,6 @@
         </div>
     </footer>
     {% block scripts %}{% endblock %}
+    <script src="https://cdn.socket.io/4.7.5/socket.io.min.js"></script>
 </body>
 </html>