P8(1): restore homepage overview curves on a memory-only sampling ring

Backend: new overviewMetrics store — a single-tier (2s x 360 = 12min)
in-memory ring fed by ServerService.RefreshStatus for exactly the eight
overview series (cpu/mem/swap/diskUsage/netUp/netDown/tcpCount/udpCount).
No persistence, no gob snapshot, no system-history page: the only surface
is GET /panel/api/server/history/:metric/:bucket restricted to those
keys, which seeds the frontend curves. ~46KB bounded by construction;
the ring starts empty on every panel start.

Frontend: restore useOverviewHistory + the VitalTile/ThroughputCard/
ConnectionsCard sparklines and the IndexPage wiring exactly as they
were before P7 removed them along with the history module.

internal/web/service/metric_history.go

@@ -87,8 +87,12 @@ type series struct {
 }
 
 func newSeries() *series {
-	s := &series{tiers: make([]*tierBuf, len(metricTiers))}
-	for i, spec := range metricTiers {
+	return newSeriesWithTiers(metricTiers)
+}
+
+func newSeriesWithTiers(specs []tierSpec) *series {
+	s := &series{tiers: make([]*tierBuf, len(specs))}
+	for i, spec := range specs {
 		s.tiers[i] = &tierBuf{resolution: spec.resolution, capacity: spec.capacity}
 	}
 	return s
@@ -112,14 +116,20 @@ func (s *series) pickTier(spanSeconds int64) *tierBuf {
 }
 
 // metricHistory is a thread-safe, in-memory store of tiered series keyed by
-// arbitrary strings. One singleton lives below: per-node metrics.
+// arbitrary strings. Two singletons live below: per-node metrics (full
+// rollup ladder) and the homepage overview series (one small fixed ring).
 type metricHistory struct {
 	mu     sync.Mutex
+	tiers  []tierSpec
 	series map[string]*series
 }
 
 func newMetricHistory() *metricHistory {
-	return &metricHistory{series: map[string]*series{}}
+	return newMetricHistoryWithTiers(metricTiers)
+}
+
+func newMetricHistoryWithTiers(tiers []tierSpec) *metricHistory {
+	return &metricHistory{tiers: tiers, series: map[string]*series{}}
 }
 
 // append stores a single sample for the given metric across all tiers.
@@ -128,7 +138,7 @@ func (h *metricHistory) append(metric string, t time.Time, v float64) {
 	defer h.mu.Unlock()
 	s := h.series[metric]
 	if s == nil {
-		s = newSeries()
+		s = newSeriesWithTiers(h.tiers)
 		h.series[metric] = s
 	}
 	s.add(t.Unix(), v)
@@ -208,12 +218,27 @@ func (h *metricHistory) aggregate(metric string, bucketSeconds int, maxPoints in
 	return out
 }
 
-// nodeMetrics is the remaining singleton: per-node CPU/Mem/net series fed by
-// NodeHeartbeatJob (via UpdateHeartbeat) and read by the node history panel.
-// The whole-host system series and the xray expvar series that used to live
-// here were removed with the system-history and xray-metrics features,
-// together with the gob persistence of the host series.
+// nodeMetrics holds per-node CPU/Mem/net series fed by NodeHeartbeatJob
+// (via UpdateHeartbeat) and read by the node history panel. The whole-host
+// system series and the xray expvar series that used to live here were
+// removed with the system-history and xray-metrics features, together with
+// the gob persistence of the host series.
 var nodeMetrics = newMetricHistory()
 
 // NodeMetricKeys lists the per-node metric names NodeHeartbeatJob writes.
 var NodeMetricKeys = []string{"cpu", "mem", "netUp", "netDown"}
+
+// overviewMetrics is the homepage overview store (P8): a deliberately small,
+// memory-only ring feeding the index-page sparkline seed — NOT a revival of
+// the removed system-history feature. One tier at the status cadence (2s)
+// holding 360 samples = a 12-minute window; eight series of 360 samples are
+// ~46 KB of payload, bounded by construction. Nothing is persisted: the
+// ring starts empty on every panel start and the overview curves simply
+// accumulate from there.
+var overviewMetrics = newMetricHistoryWithTiers([]tierSpec{{resolution: 2, capacity: 360}})
+
+// OverviewMetricKeys lists the host metric names ServerService writes on
+// every status sample for the homepage overview. It matches the frontend
+// useOverviewHistory SERIES_KEYS exactly; the controller validates incoming
+// history requests against this allow-list.
+var OverviewMetricKeys = []string{"cpu", "mem", "swap", "diskUsage", "netUp", "netDown", "tcpCount", "udpCount"}