feat: merged detection output + parallel model execution

- Multiple models run simultaneously on each frame via ThreadPoolExecutor
- Detections merged with IoU deduplication (same class, high overlap → keep highest conf)
- Single merged_annotated.mp4 output (not per-model videos)
- Single model still uses existing run_pipeline for backward compat
- Active jobs banner on upload page (polls every 5s)
- GPU warning when selecting 3+ models
- 62/62 tests pass

static/app.js

@@ -270,7 +270,14 @@
      Submit — enable/disable, loading state
   ------------------------------------------------ */
   function initFormSubmit() {
-    form.addEventListener('submit', function () {
+    form.addEventListener('submit', function (e) {
+      var checked = modelGrid.querySelectorAll('.model-card-check:checked');
+      if (checked.length >= 3) {
+        if (!confirm('Running ' + checked.length + ' models simultaneously may use significant GPU memory on Jetson. Continue?')) {
+          e.preventDefault();
+          return;
+        }
+      }
       if (videoPathInput.value) {
         form.action = '/upload/reuse';
       } else {
@@ -304,6 +311,27 @@
   initModelCards();
   initFormSubmit();
   loadExistingVideos();
+
+  // Poll active jobs
+  function checkActiveJobs() {
+    fetch('/api/jobs/active')
+      .then(function(res) { return res.json(); })
+      .then(function(data) {
+        var banner = document.getElementById('active-jobs-banner');
+        var text = document.getElementById('active-jobs-text');
+        if (banner && text) {
+          if (data.count > 0) {
+            banner.style.display = '';
+            text.textContent = data.count + ' job' + (data.count > 1 ? 's' : '') + ' in progress';
+          } else {
+            banner.style.display = 'none';
+          }
+        }
+      })
+      .catch(function() {});
+  }
+  checkActiveJobs();
+  setInterval(checkActiveJobs, 5000);
 })();
 
 function formatRelativeTime(timestamp) {