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

static/app.js

@@ -358,24 +358,6 @@ function initStatusPage(jobId, initialStatus) {
 
   if (!progressFill) return;
 
-  /* --- WebSocket live preview --- */
-  var socket = io();
-  socket.on('connect', function() {
-    socket.emit('join_job', {job_id: jobId});
-  });
-
-  socket.on('preview_frame', function(data) {
-    if (livePreviewImg && data.frame) {
-      livePreviewImg.src = 'data:image/jpeg;base64,' + data.frame;
-      if (previewPlaceholder) previewPlaceholder.style.display = 'none';
-      livePreviewImg.style.display = 'block';
-    }
-  });
-
-  window.addEventListener('beforeunload', function() {
-    socket.emit('leave_job', {job_id: jobId});
-  });
-
   /* --- Cancel button --- */
   if (cancelBtn) {
     cancelBtn.addEventListener('click', function() {