Redesign system power chart as stacked per-PSU area chart

- Add PSUReading struct and PSUs []PSUReading to LiveMetricSample
- Sample per-PSU input watts from IPMI SDR entity 10.x (Power Supply)
- Render stacked filled-area SVG chart (one layer per PSU, cumulative total)
- Fall back to single-line chart on systems with ≤1 PSU in SDR

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

audit/internal/webui/task_report.go

@@ -171,21 +171,17 @@ func renderTaskChartSVG(path string, samples []platform.LiveMetricSample, timeli
 		}
 		return gpuDisplayLabel(idx) + " Overview", buf, true
 	}
-	datasets, names, labels, title, yMin, yMax, ok := chartDataFromSamples(path, samples)
+	datasets, names, labels, title, yMin, yMax, stacked, ok := chartDataFromSamples(path, samples)
 	if !ok {
 		return "", nil, false
 	}
-	buf, err := renderMetricChartSVG(
-		title,
-		labels,
-		sampleTimes(samples),
-		datasets,
-		names,
-		yMin,
-		yMax,
-		chartCanvasHeightForPath(path, len(names)),
-		timeline,
-	)
+	var buf []byte
+	var err error
+	if stacked {
+		buf, err = renderStackedMetricChartSVG(title, labels, sampleTimes(samples), datasets, names, yMax, chartCanvasHeightForPath(path, len(names)), timeline)
+	} else {
+		buf, err = renderMetricChartSVG(title, labels, sampleTimes(samples), datasets, names, yMin, yMax, chartCanvasHeightForPath(path, len(names)), timeline)
+	}
 	if err != nil {
 		return "", nil, false
 	}