perf: throttle tracker to every 2nd frame, sup interval 10, 720p output option

- detect_interval=2 on both pipelines; stabilizer 10-frame hold bridges
  skipped frames (GPU inference cut ~half during active batch)
- sup_det_interval 5 -> 10 (still <= stabilizer hold / synthetic max_age)
- preview_every_n default 2 -> 5
- AnnotatedVideoWriter: codec auto-chain gstreamer_nvenc -> avc1 -> mp4v,
  max_height downscale (even dims, INTER_AREA); backend logged
- output_max_height plumbed Job -> web checkbox (720p) -> CLI --output-height
- README: 720p option, CLI flag
- 153 tests pass (+13)

app.py

@@ -129,6 +129,7 @@ def upload():
         n += 1
     video.save(video_path)
 
+    output_height = request.form.get("output_height", "")
     zone_mode = request.form.get("zone_mode", "dynamic")
     fixed_zone = None
     zone_preset = ""
@@ -151,6 +152,7 @@ def upload():
         zone_mode=zone_mode,
         fixed_zone=fixed_zone,
         zone_preset=zone_preset,
+        output_max_height=(int(output_height) if output_height else None),
     )
     return redirect(url_for("status", job_id=job.job_id))
 
@@ -166,6 +168,7 @@ def upload_reuse():
     if not model_configs:
         return "No models selected", 400
 
+    output_height = request.form.get("output_height", "")
     zone_mode = request.form.get("zone_mode", "dynamic")
     fixed_zone = None
     zone_preset = ""
@@ -188,6 +191,7 @@ def upload_reuse():
         zone_mode=zone_mode,
         fixed_zone=fixed_zone,
         zone_preset=zone_preset,
+        output_max_height=(int(output_height) if output_height else None),
     )
     return redirect(url_for("status", job_id=job.job_id))