Expire stale firing alerts

A resolved webhook was the only path out of the firing state, so a
notification that was dropped, silenced, or lost to a restart pinned an
alert as firing forever — Prometheus showed it resolved while
terdut-server kept listing it. The archiver only ever touched resolved
alerts, and both the list and stats queries compared status with plain
equality, so a stale row was indistinguishable from a live one.

A sweeper pass now resolves firing alerts on either of two signals: the
ends_at watermark Alertmanager sets on outgoing firing notifications has
passed (plus a grace period for clock skew), or no webhook has refreshed
the alert within TERDUT_STALE_AFTER (default 6h, above Alertmanager's 4h
repeat_interval). Such alerts get resolution_source = 'expiry',
distinguishing them from a real 'alertmanager' resolve.

Two related webhook bugs fixed alongside:

  - The upsert had no ordering guard, so a retried firing notification
    arriving after the resolved one resurrected the alert. Payloads for
    an older alert instance are now discarded: a stale retry carries the
    same startsAt, a genuine re-fire a newer one.
  - archived_at was never cleared on re-fire, leaving a re-fired alert
    archived and invisible in the default list.

Stats now exclude archived alerts to match the default list view; this
lowers historical firing/resolved totals.

The chart exposes both sweeper durations via sweeper.staleAfter and
sweeper.archiveAfter.

cmd/terdut/main.go

@@ -41,7 +41,7 @@ func main() {
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
 	defer stop()
 
-	go api.StartArchiver(ctx, database, cfg.ArchiveAfter)
+	go api.StartArchiver(ctx, database, cfg.ArchiveAfter, cfg.StaleAfter)
 
 	go func() {
 		log.Printf("terdut-server %s listening on %s", version, cfg.Addr)