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

pyproject.toml

@@ -18,7 +18,6 @@ dependencies = [
 
 [project.optional-dependencies]
 dev = ["pytest"]
-web = ["flask-socketio>=5.3.0", "eventlet>=0.33.0"]
 
 [project.scripts]
 recounter = "cli:main"