Notice when the Watchdog alert stops arriving

Everything this server does assumes alerts arrive. If Prometheus stops
evaluating, or Alertmanager cannot reach us, nothing arrives — and
silence is indistinguishable from everything being fine. The cluster has
shipped the alert for exactly this case all along: Watchdog is
expr: vector(1), so it fires permanently and is re-sent forever, and it
is worth nothing unless something downstream notices it stop. Nothing
did. It arrived, opened no incident because a repeat_interval re-send is
not a new occurrence, and when the monitoring stack died the sweeper
quietly expired it and paged nobody.

So the handling is inverted for a configurable set of alerts: receiving
one opens no incident, and the absence of one does. TERDUT_DEADMAN_MATCHERS
selects them as label matchers, defaulting to alertname=Watchdog.

The unit of monitoring is the fingerprint rather than the alert name. Two
clusters sending the same Watchdog are two independent switches, so a
healthy one can never mask a dead one. Every matcher must name an
alertname, which keeps the sweeper's candidate query on alerts_name_idx
instead of JSON-extracting labels from every row, and leaves matching
with a single implementation.

A switch is dormant until its first heartbeat: a matcher nothing has ever
sent opens nothing, so a fresh deploy or a restored database does not
page. Resolving the incident by hand sticks, exactly as it does for an
alert-backed one, so a decommissioned source is a one-time page rather
than a nag; the switch re-arms only when the heartbeat comes back, and
dying again is a new incident.

The incident has no member alerts on purpose. Linking the heartbeat would
have the settled-incident cascade close it on the very sweep that opened
it, and there is no alert describing the problem anyway — the problem is
that no alert arrived. What happened is on the timeline instead, and
recovery is the only automatic way out.

One narrow exemption in the ingest guard makes recovery possible at all.
A heartbeat we declared dead is marked resolved, and the one that proves
us wrong carries the unchanged startsAt of an alert that never stopped
firing — so "resolution is terminal within an instance" would discard it
forever and a switch could die exactly once. The exemption is scoped to
resolution_source = 'deadman', which is the only resolution this server
infers from silence on a timeout of its own, so nothing another writer
set can be undone by a stale retry. Matched alerts are also held back
from the generic staleness expiry, which would otherwise resolve a
heartbeat as 'expiry' long before its own tighter deadline.

The timeout points the opposite way to TERDUT_STALE_AFTER: staleness is a
generous grace period around a repeat_interval you do not control, while
this is a deadline you set deliberately and configure the heartbeat's
route to beat. Inheriting a 4h or 12h repeat_interval gives a dead man's
switch with a twelve hour fuse, so the README spells out the route the
heartbeat needs.

internal/api/incidents_test.go

@@ -613,7 +613,7 @@ func TestSweeper_ArchivesResolvedIncidents(t *testing.T) {
 
 	s.exec(t, "UPDATE incidents SET resolved_at = ? WHERE id = 1",
 		time.Now().Add(-30*24*time.Hour).Unix())
-	api.Sweep(context.Background(), s.db, 7*24*time.Hour, 6*time.Hour)
+	api.Sweep(context.Background(), s.db, 7*24*time.Hour, 6*time.Hour, s.deadman, s.notify)
 
 	if inc := getIncident(t, s, 1); inc["archived_at"] == nil {
 		t.Error("expected the sweeper to archive a long-resolved incident")