Put the cluster first in an ntfy page title

A phone's lock screen cuts a long title off at the end, and an incident title
keeps its grouping labels there: "PodRestarting (cluster=prod-eu,
namespace=shop)". For a team with a Kubernetes cluster per alert source, the
cluster is the first thing a person wants and the first thing lost.

When the incident has a `cluster` group label, the title of the page now
leads with it, "[prod-eu] PodRestarting (namespace=shop)", and the label is
dropped from the parenthesis so it is not said twice, along with the
parenthesis itself if nothing else is left. The reminder and the resolution
use the same title, and the label is the one the web UI's chip reads. The
message body is unchanged.

An incident without a cluster label gets the same title as before, and
incidents that are already open keep theirs; only pages sent from now on
change. The constant originLabel names the label, as ORIGIN_LABEL does in the
web UI.

No endpoint or JSON shape changed, so nothing to mirror in terdut-tui.

internal/api/notify_test.go

@@ -1,6 +1,7 @@
 package api_test
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"fmt"
@@ -164,10 +165,84 @@ func fireCritical(t *testing.T, s *ts) {
 	}, "{}:{alertname=\"DiskFull\"}")
 }
 
+// fireGrouped posts one critical alert whose Alertmanager group carries the
+// given labels, the way group_by puts them on the webhook.
+func fireGrouped(t *testing.T, s *ts, groupLabels map[string]string, groupKey string) {
+	t.Helper()
+	payload := map[string]any{
+		"version": "4", "status": "firing", "groupKey": groupKey, "groupLabels": groupLabels,
+		"alerts": []map[string]any{amAlert("fp-grouped", "PodRestarting", "firing",
+			"2026-05-20T10:00:00Z", zeroTime, map[string]string{"severity": "critical"})},
+	}
+	data, _ := json.Marshal(payload)
+	resp, err := http.Post(s.URL+"/api/integrations/"+s.ingestKey+"/alertmanager",
+		"application/json", bytes.NewReader(data))
+	if err != nil {
+		t.Fatalf("post webhook: %v", err)
+	}
+	resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("webhook returned %d", resp.StatusCode)
+	}
+}
+
 // ---------------------------------------------------------------------------
 // Delivery
 // ---------------------------------------------------------------------------
 
+// A phone cuts a long title off at the end, and the incident title keeps the
+// grouping labels there, so the cluster leads the page instead.
+func TestNotify_ClusterLeadsTheTitle(t *testing.T) {
+	s, f := notifyTS(t, api.NotifyConfig{PublicURL: "https://terdut.example.com"})
+
+	fireGrouped(t, s, map[string]string{
+		"alertname": "PodRestarting", "cluster": "prod-eu", "namespace": "shop",
+	}, `{}:{alertname="PodRestarting",cluster="prod-eu",namespace="shop"}`)
+	s.sweepNotify(t)
+
+	msgs := f.messages()
+	if len(msgs) != 1 {
+		t.Fatalf("expected 1 push, got %d", len(msgs))
+	}
+	if want := "[prod-eu] PodRestarting (namespace=shop)"; msgs[0].Title != want {
+		t.Errorf("title = %q, want %q", msgs[0].Title, want)
+	}
+}
+
+// Only the cluster is the whole grouping: no parenthesis is left behind.
+func TestNotify_ClusterAloneLeavesNoParenthesis(t *testing.T) {
+	s, f := notifyTS(t, api.NotifyConfig{})
+
+	fireGrouped(t, s, map[string]string{"alertname": "PodRestarting", "cluster": "prod-eu"},
+		`{}:{alertname="PodRestarting",cluster="prod-eu"}`)
+	s.sweepNotify(t)
+
+	msgs := f.messages()
+	if len(msgs) != 1 {
+		t.Fatalf("expected 1 push, got %d", len(msgs))
+	}
+	if want := "[prod-eu] PodRestarting"; msgs[0].Title != want {
+		t.Errorf("title = %q, want %q", msgs[0].Title, want)
+	}
+}
+
+// Nothing changes for a team whose alerts have no cluster label.
+func TestNotify_NoClusterKeepsTheTitle(t *testing.T) {
+	s, f := notifyTS(t, api.NotifyConfig{})
+
+	fireGrouped(t, s, map[string]string{"alertname": "PodRestarting", "namespace": "shop"},
+		`{}:{alertname="PodRestarting",namespace="shop"}`)
+	s.sweepNotify(t)
+
+	msgs := f.messages()
+	if len(msgs) != 1 {
+		t.Fatalf("expected 1 push, got %d", len(msgs))
+	}
+	if want := "PodRestarting (namespace=shop)"; msgs[0].Title != want {
+		t.Errorf("title = %q, want %q", msgs[0].Title, want)
+	}
+}
+
 func TestNotify_TriggeredIncidentPagesOnCall(t *testing.T) {
 	s, f := notifyTS(t, api.NotifyConfig{PublicURL: "https://terdut.example.com"})