Set the chart's placeholder version to 0.42.1

Cosmetic: make helm-package passes --version and --app-version from the
tag, so these two fields decide nothing about what gets published. Still
done, as with def0f68 (0.42.0) and b848143 (0.41.3) before it, because a
tree heading for v0.42.1 that still says 0.42.0 tells its reader
something false.

charts/terdut-server/Chart.yaml

@@ -15,5 +15,5 @@ type: application
 # appVersion and image.tag in values.yaml no longer agree, and that is not an oversight:
 # image.tag stays "latest", which is what a local install actually pulls. appVersion is
 # metadata and drives nothing.
-version: 0.42.0
-appVersion: "v0.42.0"
+version: 0.42.1
+appVersion: "v0.42.1"

internal/api/notifier.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"log"
 	"net/http"
+	"regexp"
 	"strings"
 	"time"
 
@@ -409,9 +410,11 @@ func renderNotification(inc models.Incident, n outboxRow, firing int, cfg Notify
 			strings.TrimSuffix(cfg.PublicURL, "/"), inc.ID)
 	}
 
+	title := pageTitle(inc)
+
 	switch n.kind {
 	case notifyResolved:
-		msg.Title = "Resolved: " + inc.Title
+		msg.Title = "Resolved: " + title
 		msg.Message = "All alerts stopped firing after " +
 			humanDuration(time.Since(inc.TriggeredAt))
 		msg.Priority = ntfyPriorityLow
@@ -419,9 +422,9 @@ func renderNotification(inc models.Incident, n outboxRow, firing int, cfg Notify
 		return msg
 
 	case notifyReminder:
-		msg.Title = "Still unacknowledged: " + inc.Title
+		msg.Title = "Still unacknowledged: " + title
 	default:
-		msg.Title = inc.Title
+		msg.Title = title
 	}
 
 	severity := derefString(inc.Severity)
@@ -443,6 +446,48 @@ func renderNotification(inc models.Incident, n outboxRow, firing int, cfg Notify
 	return msg
 }
 
+// originLabel is the label that says where an alert came from, for a team with
+// several Kubernetes clusters behind it. It comes from Prometheus's
+// externalLabels and reaches an incident through Alertmanager's group_by; the
+// web UI reads the same label, and the README ("Several clusters, one team")
+// explains how to set it up.
+const originLabel = "cluster"
+
+// pageTitle is the incident's title for a notification. A phone's lock screen
+// cuts a long title off at the end, and the incident title puts the grouping
+// labels there, so the cluster would be the first thing lost. When the incident
+// has an origin it leads instead, "[prod-eu] PodRestarting (namespace=foo)", and
+// is dropped from the parenthesis so it is not said twice. A title that is not
+// in incidentTitle's "name (k=v, k=v)" shape keeps its text and gains the prefix.
+func pageTitle(inc models.Incident) string {
+	origin := inc.GroupLabels[originLabel]
+	if origin == "" {
+		return inc.Title
+	}
+	return "[" + origin + "] " + titleWithoutLabel(inc.Title, originLabel, origin)
+}
+
+var titleShape = regexp.MustCompile(`(?s)^(.*?) \((.*)\)$`)
+
+// titleWithoutLabel removes "key=value" from the parenthesised tail of a title
+// built by incidentTitle, and the parentheses with it if nothing else is left.
+func titleWithoutLabel(title, key, value string) string {
+	m := titleShape.FindStringSubmatch(title)
+	if m == nil {
+		return title
+	}
+	var rest []string
+	for _, part := range strings.Split(m[2], ", ") {
+		if part != key+"="+value {
+			rest = append(rest, part)
+		}
+	}
+	if len(rest) == 0 {
+		return m[1]
+	}
+	return m[1] + " (" + strings.Join(rest, ", ") + ")"
+}
+
 // ntfy's priority scale. Max is the one that overrides the phone's quiet
 // settings, which is the whole point of paging on critical.
 const (

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"})