feat(admin): System health page surfacing stuck/failed emails

New /admin/system-health page (sidebar entry, settings.view) that lists
emails the queue gave up on (status='failed' or pending+retry>=3) with
retry (re-queue) and dismiss (delete) actions. Backend adds /failures,
/failures/email/:id/retry and DELETE on adminSystemHealth. First source
is email failures (the original trigger — quote_sent template errors
left invoices unsent for 14h with no signal); more sources can be added.

frontend/src/App.tsx

@@ -21,6 +21,7 @@ import {
   ArchivesPage,
   AnalyticsPage,
   SettingsPage,
+  SystemHealthPage,
   UserManagementPage,
   CustomerManagementPage,
   CustomerDetailPage,
@@ -267,6 +268,7 @@ function App() {
                       <Route path="customers/:id" element={<RedirectCustomerDetail />} />
 
                       <Route path="settings" element={<SettingsPage />} />
+                      <Route path="system-health" element={<SystemHealthPage />} />
                       <Route path="webhooks/:id/deliveries" element={<WebhookDeliveriesPage />} />
 
                       {/* Old top-level routes — these surfaces now live as