Show notes from similar earlier incidents

Each incident gets a signature: the alert name plus the group labels that
say what is broken, minus the ones that only say where it ran (instance,
pod, container, ...). GET /api/incidents/{id}/similar returns resolved
incidents in the same team with the same signature that have notes.

Notes can be marked as the resolution note, "what fixed it", either with a
resolution field on resolve or pinned on a note. Those lead the similar
list, show on the incident page as "Seen before", and the triggered
notification carries the latest one.

Claude-Session: https://claude.ai/code/session_01MMados3BD1oSjevHxbmVqU

internal/api/incident_store.go

@@ -36,6 +36,10 @@ const (
 	evUnsnoozed      = "unsnoozed"
 	evResolved       = "resolved"
 	evNote           = "note"
+	// evResolutionNote is the note worth finding again: what fixed it. The
+	// similar-incidents lookup and the page lead with these; plain notes are
+	// the working chatter and stay one click away.
+	evResolutionNote = "resolution_note"
 	evDeadmanSilent  = "deadman_silent"
 )
 
@@ -295,6 +299,34 @@ func openIncidentForAlert(ctx context.Context, q querier, alertID int64) (int64,
 	return id, err
 }
 
+// volatileLabels say where a problem ran this time, not what the problem is, so
+// they stay out of the signature. Migration 008's backfill lists the same set.
+var volatileLabels = map[string]bool{
+	"instance": true, "pod": true, "pod_name": true, "pod_ip": true,
+	"container": true, "container_name": true, "endpoint": true,
+}
+
+// incidentSignature identifies "the same problem" across incidents: the alert
+// name plus the stable group labels, sorted. Incidents in one team with equal
+// signatures are what the similar-incidents lookup returns. title stands in for
+// the name when the payload carried no alertname (groupless and dead man's
+// switch incidents).
+func incidentSignature(groupLabels map[string]string, title string) string {
+	name := groupLabels["alertname"]
+	if name == "" {
+		name = title
+	}
+	rest := make([]string, 0, len(groupLabels))
+	for k, v := range groupLabels {
+		if k == "alertname" || volatileLabels[k] {
+			continue
+		}
+		rest = append(rest, k+"="+v)
+	}
+	sort.Strings(rest)
+	return name + "|" + strings.Join(rest, ",")
+}
+
 // incidentTitle renders a human-readable title from Alertmanager's groupLabels,
 // leading with the alert name and appending whatever else the operator grouped
 // by. Falls back to the alert's own name when the payload carried no groupLabels.