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_test.go

@@ -969,8 +969,8 @@ func TestTaskChartSVGUsesTaskTimeWindow(t *testing.T) {
 		{Timestamp: base.Add(-1 * time.Minute), PowerW: 300},
 	}
 	for _, sample := range samples {
-		if err := db.Write(sample); err != nil {
-			t.Fatalf("Write: %v", err)
+		if err := db.writeAt(sample.Timestamp.Unix(), sample); err != nil {
+			t.Fatalf("writeAt: %v", err)
 		}
 	}
 	_ = db.Close()