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/metricsdb_test.go

@@ -153,11 +153,12 @@ func TestMetricsDBLoadBetweenFiltersWindow(t *testing.T) {
 
 	base := time.Unix(1_700_000_000, 0).UTC()
 	for i := 0; i < 5; i++ {
-		if err := db.Write(platform.LiveMetricSample{
-			Timestamp:  base.Add(time.Duration(i) * time.Minute),
+		ts := base.Add(time.Duration(i) * time.Minute)
+		if err := db.writeAt(ts.Unix(), platform.LiveMetricSample{
+			Timestamp:  ts,
 			CPULoadPct: float64(i),
 		}); err != nil {
-			t.Fatalf("Write(%d): %v", i, err)
+			t.Fatalf("writeAt(%d): %v", i, err)
 		}
 	}