feat(metrics): SQLite persistence + chart fixes (no dots, peak label, min/avg/max in title)

- Add modernc.org/sqlite dependency; write every sample to
  /appdata/bee/metrics.db (WAL mode, prune to 24h on startup)
- Pre-fill ring buffers from last 120 DB rows on startup so charts
  survive service restarts
- Ticker changed 3s→1s; chart JS refresh will be set to 2s (lag ≤3s)
- Add GET /api/metrics/export.csv for full history download
- Chart rendering: SymbolNone (no dots), right padding=80px so peak
  mark line label is not clipped, min/avg/max appended to chart title

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

audit/internal/webui/server.go

@@ -132,6 +132,8 @@ type handler struct {
 	// per-GPU rings (index = GPU index)
 	gpuRings []*gpuRings
 	ringsMu  sync.Mutex
+	// metrics persistence (nil if DB unavailable)
+	metricsDB *MetricsDB
 	// install job (at most one at a time)
 	installJob *jobState
 	installMu  sync.Mutex
@@ -158,6 +160,18 @@ func NewHandler(opts HandlerOptions) http.Handler {
 		ringMemLoad: newMetricsRing(120),
 		ringPower:   newMetricsRing(120),
 	}
+
+	// Open metrics DB and pre-fill ring buffers from history.
+	if db, err := openMetricsDB(metricsDBPath); err == nil {
+		h.metricsDB = db
+		db.Prune(metricsKeepDuration)
+		if samples, err := db.LoadRecent(120); err == nil {
+			for _, s := range samples {
+				h.feedRings(s)
+			}
+		}
+	}
+
 	globalQueue.startWorker(&opts)
 	mux := http.NewServeMux()
 
@@ -231,9 +245,10 @@ func NewHandler(opts HandlerOptions) http.Handler {
 	mux.HandleFunc("POST /api/install/run", h.handleAPIInstallRun)
 	mux.HandleFunc("GET /api/install/stream", h.handleAPIInstallStream)
 
-	// Metrics — SSE stream of live sensor data + server-side SVG charts
+	// Metrics — SSE stream of live sensor data + server-side SVG charts + CSV export
 	mux.HandleFunc("GET /api/metrics/stream", h.handleAPIMetricsStream)
 	mux.HandleFunc("GET /api/metrics/chart/", h.handleMetricsChartSVG)
+	mux.HandleFunc("GET /api/metrics/export.csv", h.handleAPIMetricsExportCSV)
 
 	// Reanimator chart static assets (viewer template expects /static/*)
 	mux.Handle("GET /static/", http.StripPrefix("/static/", web.Static()))
@@ -618,6 +633,16 @@ func renderChartSVG(title string, datasets [][]float64, names []string, labels [
 			datasets[i] = make([]float64, n)
 		}
 	}
+	// Append global min/avg/max to title.
+	mn, avg, mx := globalStats(datasets)
+	if mx > 0 {
+		title = fmt.Sprintf("%s    ↓%s  ~%s  ↑%s",
+			title,
+			chartLegendNumber(mn),
+			chartLegendNumber(avg),
+			chartLegendNumber(mx),
+		)
+	}
 	title = sanitizeChartText(title)
 	names = sanitizeChartTexts(names)
 	sparse := sanitizeChartTexts(sparseLabels(labels, 6))
@@ -626,6 +651,9 @@ func renderChartSVG(title string, datasets [][]float64, names []string, labels [
 	opt.Title = gocharts.TitleOption{Text: title}
 	opt.XAxis.Labels = sparse
 	opt.Legend = gocharts.LegendOption{SeriesNames: names}
+	opt.Symbol = gocharts.SymbolNone
+	// Right padding: reserve space for the MarkLine label (library recommendation).
+	opt.Padding = gocharts.NewBox(20, 80, 20, 20)
 	if yMin != nil || yMax != nil {
 		opt.YAxis = []gocharts.YAxisOption{{
 			Min:            yMin,
@@ -635,8 +663,8 @@ func renderChartSVG(title string, datasets [][]float64, names []string, labels [
 	}
 
 	// Add a single peak mark line on the series that holds the global maximum.
-	peakIdx, peakVal := globalPeakSeries(datasets)
-	if peakIdx >= 0 && peakVal > 0 && peakIdx < len(opt.SeriesList) {
+	peakIdx, _ := globalPeakSeries(datasets)
+	if peakIdx >= 0 && peakIdx < len(opt.SeriesList) {
 		opt.SeriesList[peakIdx].MarkLine = gocharts.NewMarkLine(gocharts.SeriesMarkTypeMax)
 	}
 
@@ -666,6 +694,33 @@ func globalPeakSeries(datasets [][]float64) (idx int, peak float64) {
 	return idx, peak
 }
 
+// globalStats returns min, average, and max across all values in all datasets.
+func globalStats(datasets [][]float64) (mn, avg, mx float64) {
+	var sum float64
+	var count int
+	first := true
+	for _, ds := range datasets {
+		for _, v := range ds {
+			if first {
+				mn, mx = v, v
+				first = false
+			}
+			if v < mn {
+				mn = v
+			}
+			if v > mx {
+				mx = v
+			}
+			sum += v
+			count++
+		}
+	}
+	if count > 0 {
+		avg = sum / float64(count)
+	}
+	return mn, avg, mx
+}
+
 func sanitizeChartText(s string) string {
 	if s == "" {
 		return ""
@@ -747,6 +802,17 @@ func sparseLabels(labels []string, n int) []string {
 	return out
 }
 
+func (h *handler) handleAPIMetricsExportCSV(w http.ResponseWriter, r *http.Request) {
+	if h.metricsDB == nil {
+		http.Error(w, "metrics database not available", http.StatusServiceUnavailable)
+		return
+	}
+	w.Header().Set("Content-Type", "text/csv; charset=utf-8")
+	w.Header().Set("Content-Disposition", `attachment; filename="bee-metrics.csv"`)
+	w.Header().Set("Cache-Control", "no-store")
+	_ = h.metricsDB.ExportCSV(w)
+}
+
 // ── Page handler ─────────────────────────────────────────────────────────────
 
 func (h *handler) handleReady(w http.ResponseWriter, r *http.Request) {