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/hooks/admin/useCombinedAlerts.ts

@@ -0,0 +1,49 @@
+import { useQuery } from '@tanstack/react-query';
+import { useSystemAlerts } from '@/hooks/useSystemHealth';
+import { useUnresolvedAlerts } from '@/hooks/useRateLimitAlerts';
+import { queryKeys } from '@/lib/queryKeys';
+
+export interface CombinedAlert {
+  id: string;
+  created_at: string;
+  severity: 'critical' | 'high' | 'medium' | 'low';
+  message: string;
+  alert_type?: string;
+  source: 'system' | 'rate_limit';
+  resolved_at?: string | null;
+  metric_type?: string;
+  function_name?: string;
+}
+
+export function useCombinedAlerts() {
+  const systemCritical = useSystemAlerts('critical');
+  const systemHigh = useSystemAlerts('high');
+  const rateLimitAlerts = useUnresolvedAlerts();
+
+  return useQuery({
+    queryKey: queryKeys.monitoring.combinedAlerts(),
+    queryFn: () => {
+      const combined: CombinedAlert[] = [
+        ...(systemCritical.data || []).map(a => ({ ...a, source: 'system' as const })),
+        ...(systemHigh.data || []).map(a => ({ ...a, source: 'system' as const })),
+        ...(rateLimitAlerts.data || []).map(a => ({ 
+          id: a.id,
+          created_at: a.created_at,
+          severity: 'high' as const, // Rate limit alerts are considered high severity
+          message: a.alert_message,
+          alert_type: a.metric_type,
+          source: 'rate_limit' as const,
+          resolved_at: a.resolved_at,
+          metric_type: a.metric_type,
+          function_name: a.function_name,
+        })),
+      ];
+      return combined
+        .sort((a, b) => new Date(b.created_at).getTime() - new Date(a.created_at).getTime())
+        .slice(0, 10);
+    },
+    enabled: !systemCritical.isLoading && !systemHigh.isLoading && !rateLimitAlerts.isLoading,
+    staleTime: 15000,
+    refetchInterval: 30000,
+  });
+}