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/config/config.go

@@ -9,6 +9,11 @@ type Config struct {
 	Addr         string
 	DBPath       string
 	ArchiveAfter time.Duration
+
+	// StaleAfter is how long a firing alert may go without a refreshing webhook
+	// before the sweeper treats it as resolved. It must exceed Alertmanager's
+	// repeat_interval (default 4h), which is what refreshes the alert.
+	StaleAfter time.Duration
 }
 
 func Load() Config {
@@ -26,5 +31,11 @@ func Load() Config {
 			archiveAfter = d
 		}
 	}
-	return Config{Addr: addr, DBPath: dbPath, ArchiveAfter: archiveAfter}
+	staleAfter := 6 * time.Hour
+	if s := os.Getenv("TERDUT_STALE_AFTER"); s != "" {
+		if d, err := time.ParseDuration(s); err == nil {
+			staleAfter = d
+		}
+	}
+	return Config{Addr: addr, DBPath: dbPath, ArchiveAfter: archiveAfter, StaleAfter: staleAfter}
 }