fix(webui): make metrics history immune to system-clock changes

Every sample was stamped with time.Now(), so a timezone switch or NTP
step punched a multi-hour gap into the series: old points collapsed to
the left edge, new points bunched at the right, joined by one diagonal.

Stamp rows from a monotonic seqClock instead — seeded once from the wall
clock (or the newest persisted row) and thereafter advanced only by the
monotonic elapsed time between writes. Rebase Downsample/Prune on the
newest sample rather than time.Now() so a clock step just before the
hourly compaction can't drop fresh data.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GofKhuF9xQHaz3UFfncR6D

audit/internal/webui/server.go

@@ -393,9 +393,8 @@ func (h *handler) startMetricsCollector() {
 				h.setLatestMetric(sample)
 			case <-pruneTicker.C:
 				if h.metricsDB != nil {
-					now := time.Now().UTC()
-					_ = h.metricsDB.Downsample(now.Add(-metricsDownsampleAge), now.Add(-metricsRetainWindow))
-					_ = h.metricsDB.Prune(now.Add(-metricsRetainWindow))
+					_ = h.metricsDB.Downsample(metricsDownsampleAge, metricsRetainWindow)
+					_ = h.metricsDB.Prune(metricsRetainWindow)
 				}
 			}
 		}