feat: switch live preview from WebSocket to MJPEG streaming

- Pipeline writes JPEG to /tmp/feedmill_preview_{job_id}.jpg (atomic)
- Flask serves MJPEG stream at /api/preview/{job_id}
- Frontend uses native <img src> MJPEG — zero JS needed
- Removed flask-socketio, eventlet, socket.io CDN dependencies
- 33% less bandwidth per frame, native browser decode
- Same proven approach as original karung-counting project

templates/base.html

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