feat: live preview toggle, FPS indicator, live counting by object

- Toggle ON/OFF switch to disable preview (faster processing)
- FPS badge showing rolling average processing speed
- Live Loading/Unloading/Net counts updated in real-time
- Pipeline callback pushes stats every 10 frames
- Frontend polls and updates display every 2s
- 62/62 tests pass

app.py

@@ -279,6 +279,10 @@ def api_job_detail(job_id):
             for r in job.results
         ],
         "error": job.error,
+        "live_fps": job.live_fps,
+        "live_loading": job.live_loading,
+        "live_unloading": job.live_unloading,
+        "live_frame": job.live_frame,
     })