Fix dashboard freeze: waitress, reduce frame refresh, atomic reads

- Switch Flask dev server -> waitress (8 threads, production WSGI)
- Only refresh frame.jpg when frame_index changes, not every second
- Safety try/except on send_file for edge-case file conflicts
- Fix test_run.sh: export PYTHONPATH, use venv python
- Update service file to use venv python

templates/index.html

@@ -121,11 +121,10 @@ function selectCam(id) {
 function poll() {
     if (!activeCam) return;
     var t = Date.now();
-    var img = document.getElementById("frame-img");
-    img.src = "/shm/" + activeCam + "/frame.jpg?t=" + t;
     fetch("/shm/" + activeCam + "/stats.json?t=" + t).then(r => r.ok ? r.json() : null).then(s => {
         if (!s) { setOffline(); return; }
         lastUpdate = Date.now();
+        var oldFrame = lastFramePerCam[activeCam] || 0;
         lastFramePerCam[activeCam] = s.frame_index;
         perCam[activeCam] = { total: s.total_entered_count, inside: s.inside_box_count, frame: s.frame_index, tracks: s.track_count, speed: s.smoothed_speed };
         document.getElementById("s-total").textContent = s.total_entered_count;
@@ -138,6 +137,10 @@ function poll() {
         tag.textContent = activeCam + " \u2022 " + status;
         tag.className = "tag " + (s.backward_active ? "off" : "live");
         document.getElementById("cam-count").textContent = s.total_entered_count;
+        // only refresh frame when index changes (every 5 frames with interval_frames=5)
+        if (s.frame_index != oldFrame) {
+            document.getElementById("frame-img").src = "/shm/" + activeCam + "/frame.jpg?t=" + t;
+        }
     });
 }