feat(ui/alerts): register /alerts/* routes with placeholder pages

Adds 6 lazy-loaded route entries for the alerting UI (Inbox, All, History,
Rules list, Rule editor wizard, Silences) plus an `/alerts` → `/alerts/inbox`
redirect. Page components are placeholder stubs to be replaced in Phase 5/6/7.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/src/pages/Alerts/SilencesPage.tsx

@@ -0,0 +1,3 @@
+export default function SilencesPage() {
+  return <div>SilencesPage — coming soon</div>;
+}