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.

internal/api/alerts.go

@@ -21,7 +21,7 @@ const alertSelectFrom = `
 	       a.labels, a.annotations,
 	       a.starts_at, a.ends_at, a.generator_url, a.received_at,
 	       a.acknowledged_by, a.acknowledged_at, u.username,
-	       a.archived_at
+	       a.resolution_source, a.archived_at
 	FROM   alerts a
 	LEFT   JOIN users u ON u.id = a.acknowledged_by`
 
@@ -186,7 +186,7 @@ func scanAlert(s scanner) (models.Alert, error) {
 		&startsAtUnix, &endsAtUnix,
 		&a.GeneratorURL, &receivedAtUnix,
 		&ackByID, &ackAtUnix, &ackByUser,
-		&archivedAtUnix,
+		&a.ResolutionSource, &archivedAtUnix,
 	); err != nil {
 		return a, err
 	}