Implement admin database stats dashboard

Add admin-only database statistics dashboard
- Introduces types for database statistics and recent additions
- Implements hooks to fetch statistics and recent additions via RPCs
- Adds UI components for stats cards and a recent additions table
- Integrates new AdminDatabaseStats page and routing under /admin/database-stats
- Updates admin sidebar and app routes to expose the new dashboard
- Enables real-time updates and export capabilities for recent additions

src/lib/queryKeys.ts

@@ -98,4 +98,10 @@ export const queryKeys = {
     anomalyDetections: () => ['monitoring', 'anomaly-detections'] as const,
     dataRetentionStats: () => ['monitoring', 'data-retention-stats'] as const,
   },
+  
+  // Admin queries
+  admin: {
+    databaseStats: () => ['admin', 'database-stats'] as const,
+    recentAdditions: (limit: number) => ['admin', 'recent-additions', limit] as const,
+  },
 } as const;