Show which cluster an incident came from, as a chip

A team with one Alertmanager per Kubernetes cluster could not tell at a
glance where an incident started: the cluster was only a word inside the
title. When an alert carries a `cluster` label, the queue rows, the incident
page and the alerts list now show it as a chip in that cluster's colour, a
stable pick from the existing six-colour palette. A queue row drops
`cluster=...` from its title, since the chip says it, and the incident page
keeps the full title.

An incident has the label only when it is in Alertmanager's group_by, which
is also what keeps two clusters' identical alerts apart: incidents are matched
on the team and the groupKey, and the groupKey does not include external
labels. The README has a section on the two settings (Prometheus
externalLabels and group_by). The alerts list reads the label from the alert
itself, so it shows the chip with only the external label.

Web UI and docs only: no endpoint or JSON shape changed, so nothing to mirror
in terdut-tui. Nothing changes for a team whose alerts have no cluster label.

internal/web/static/js/incident.js

@@ -4,10 +4,10 @@
 import * as api from './api.js';
 import * as poll from './poll.js';
 import {
-  h, clear, icon, badge, severityBadge, labelChip, openSheet, closeSheet, confirm, toast, spinner, emptyState,
+  h, clear, icon, badge, severityBadge, originChip, labelChip, openSheet, closeSheet, confirm, toast, spinner, emptyState,
 } from './ui.js';
 import {
-  ago, when, until, duration, isFuture, STATUS_LABEL,
+  ago, when, until, duration, isFuture, originOf, STATUS_LABEL,
 } from './format.js';
 import { myID, users } from './state.js';
 import { back } from './app.js';
@@ -99,6 +99,8 @@ function render() {
 
 function statusBadges() {
   const out = [];
+  const origin = originOf(inc.group_labels);
+  if (origin) out.push(originChip(origin));
   if (inc.severity) out.push(severityBadge(inc.severity));
   out.push(badge(STATUS_LABEL[inc.status] || inc.status, `st-${inc.status}`));
   if (inc.status !== 'resolved' && isFuture(inc.snoozed_until)) {