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>
