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/AllAlertsPage.tsx

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

ui/src/pages/Alerts/HistoryPage.tsx

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+export default function HistoryPage() {
+  return <div>HistoryPage — coming soon</div>;
+}

ui/src/pages/Alerts/InboxPage.tsx

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+export default function InboxPage() {
+  return <div>InboxPage — coming soon</div>;
+}

ui/src/pages/Alerts/RuleEditor/RuleEditorWizard.tsx

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+export default function RuleEditorWizard() {
+  return <div>RuleEditorWizard — coming soon</div>;
+}

ui/src/pages/Alerts/RulesListPage.tsx

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+export default function RulesListPage() {
+  return <div>RulesListPage — coming soon</div>;
+}

ui/src/pages/Alerts/SilencesPage.tsx

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+export default function SilencesPage() {
+  return <div>SilencesPage — coming soon</div>;
+}