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/lib/queryKeys.ts

@@ -77,4 +77,17 @@ export const queryKeys = {
   lists: {
     items: (listId: string) => ['list-items', listId] as const,
   },
+  
+  // Monitoring queries
+  monitoring: {
+    overview: () => ['monitoring', 'overview'] as const,
+    systemHealth: () => ['system-health'] as const,
+    systemAlerts: (severity?: string) => ['system-alerts', severity] as const,
+    rateLimitStats: (timeWindow: number) => ['rate-limit-stats', timeWindow] as const,
+    recentErrors: (timeWindow: number) => ['recent-errors', timeWindow] as const,
+    recentActivity: (timeWindow: number) => ['recent-activity', timeWindow] as const,
+    combinedAlerts: () => ['monitoring', 'combined-alerts'] as const,
+    databaseHealth: () => ['monitoring', 'database-health'] as const,
+    moderationHealth: () => ['monitoring', 'moderation-health'] as const,
+  },
 } as const;