feat(slm): point dashboard live tile at /monitor too

Finishes the live-view pivot: the SLM dashboard's live-chart tile now uses
the fan-out /monitor feed (multi-viewer, L1/L10) instead of the DRD /stream,
and skips heartbeat / unreachable frames so they don't blank the metrics or
spike the chart.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

templates/sound_level_meters.html

@@ -280,7 +280,7 @@ function startDashboardStream() {
     }
 
     const wsProtocol = window.location.protocol === 'https:' ? 'wss:' : 'ws:';
-    const wsUrl = `${wsProtocol}//${window.location.host}/api/slmm/${window.selectedUnitId}/live`;
+    const wsUrl = `${wsProtocol}//${window.location.host}/api/slmm/${window.selectedUnitId}/monitor`;
 
     window.dashboardWebSocket = new WebSocket(wsUrl);
 
@@ -293,6 +293,10 @@ function startDashboardStream() {
     window.dashboardWebSocket.onmessage = function(event) {
         try {
             const data = JSON.parse(event.data);
+            // /monitor sends keepalive 'heartbeat' frames (no metrics) and a per-frame
+            // 'feed_status'; skip heartbeats and offline frames so they don't blank the
+            // metrics or spike the chart with zeros.
+            if (data.heartbeat || data.feed_status === 'unreachable') return;
             updateDashboardMetrics(data);
             updateDashboardChart(data);
         } catch (error) {