feat(slm): backfill the live chart from the DOD trail on open

On opening the live view, fetch GET /api/slmm/{unit}/history?hours=2 and
seed the chart with the recent trend BEFORE connecting the live socket, so
it opens with context instead of blank. Live frames then append in order.

- backfillChart() populates all four series (Lp/Leq/L1/L10) from the trail.
- initLiveDataStream is async and awaits the backfill before opening the WS.
- Chart rolling window raised 60 -> 600 points so the ~2h backfill (1/min)
  isn't immediately shifted out.
- Trail timestamps are naive UTC -> append 'Z' so they localize consistently
  with the live frames.

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

templates/partials/slm_live_view.html

@@ -513,7 +513,37 @@ if (typeof window.currentWebSocket === 'undefined') {
     window.currentWebSocket = null;
 }
 
-function initLiveDataStream(unitId) {
+// Backfill the chart with the recent DOD trail so it opens with context.
+async function backfillChart(unitId) {
+    try {
+        const r = await fetch(`/api/slmm/${encodeURIComponent(unitId)}/history?hours=2`);
+        if (!r.ok) return;
+        const d = await r.json();
+        const readings = d.readings || [];
+        if (!window.chartData) return;
+        for (const row of readings) {
+            // Trail timestamps are naive UTC; append 'Z' so they convert to local
+            // consistently with the live frames (which use local Date.now()).
+            window.chartData.timestamps.push(row.timestamp ? new Date(row.timestamp + 'Z').toLocaleTimeString() : '');
+            window.chartData.lp.push(parseFloat(row.lp || 0));
+            window.chartData.leq.push(parseFloat(row.leq || 0));
+            window.chartData.ln1.push(parseFloat(row.ln1 || 0));
+            window.chartData.ln2.push(parseFloat(row.ln2 || 0));
+        }
+        if (window.liveChart) {
+            window.liveChart.data.labels = window.chartData.timestamps;
+            window.liveChart.data.datasets[0].data = window.chartData.lp;
+            window.liveChart.data.datasets[1].data = window.chartData.leq;
+            window.liveChart.data.datasets[2].data = window.chartData.ln1;
+            window.liveChart.data.datasets[3].data = window.chartData.ln2;
+            window.liveChart.update('none');
+        }
+    } catch (e) {
+        console.warn('Chart backfill failed:', e);
+    }
+}
+
+async function initLiveDataStream(unitId) {
     // Close existing connection if any
     if (window.currentWebSocket) {
         window.currentWebSocket.close();
@@ -533,6 +563,10 @@ function initLiveDataStream(unitId) {
         window.liveChart.update();
     }
 
+    // Seed the chart with recent history BEFORE opening the live socket, so live
+    // frames append after the backfill (right order) and the chart isn't blank.
+    await backfillChart(unitId);
+
     // WebSocket URL for SLMM backend via proxy.
     // /monitor = the shared fan-out DOD feed (many viewers, one device connection,
     // and it carries L1/L10 which the DRD /stream cannot).
@@ -649,8 +683,9 @@ function updateLiveChart(data) {
     window.chartData.ln1.push(parseFloat(data.ln1 || 0));
     window.chartData.ln2.push(parseFloat(data.ln2 || 0));
 
-    // Keep only last 60 data points
+    // Keep a rolling window large enough to hold the ~2h backfill (one point/min)
-    if (window.chartData.timestamps.length > 60) {
+    // plus a good run of live points before the oldest scroll off.
+    if (window.chartData.timestamps.length > 600) {
         window.chartData.timestamps.shift();
         window.chartData.lp.shift();
         window.chartData.leq.shift();