Implement monitoring overview features

Add comprehensive monitoring dashboard scaffolding:
- Extend queryKeys with monitoring keys
- Create hooks: useCombinedAlerts, useRecentActivity, useDatabaseHealth, useModerationHealth
- Build UI components: SystemHealthStatus, CriticalAlertsPanel, MonitoringQuickStats, RecentActivityTimeline, MonitoringNavCards
- Create MonitoringOverview page and integrate routing (MonitoringOverview route)
- Wire AdminSidebar to include Monitoring navigation
- Introduce supporting routing and admin layout hooks/utilities
- Align with TanStack Query patterns and plan for auto-refresh and optimistic updates

src/App.tsx

@@ -75,6 +75,7 @@ const ErrorMonitoring = lazy(() => import("./pages/admin/ErrorMonitoring"));
 const ErrorLookup = lazy(() => import("./pages/admin/ErrorLookup"));
 const TraceViewer = lazy(() => import("./pages/admin/TraceViewer"));
 const RateLimitMetrics = lazy(() => import("./pages/admin/RateLimitMetrics"));
+const MonitoringOverview = lazy(() => import("./pages/admin/MonitoringOverview"));
 
 // User routes (lazy-loaded)
 const Profile = lazy(() => import("./pages/Profile"));
@@ -405,6 +406,14 @@ function AppContent(): React.JSX.Element {
                       </AdminErrorBoundary>
                     } 
                   />
+                  <Route 
+                    path="/admin/monitoring-overview" 
+                    element={
+                      <AdminErrorBoundary section="Monitoring Overview">
+                        <MonitoringOverview />
+                      </AdminErrorBoundary>
+                    } 
+                  />
                   
                   {/* Utility routes - lazy loaded */}
                   <Route path="/force-logout" element={<ForceLogout />} />