Refactor user growth chart data model and enhance demo user creation

- Renamed `totalUsers` and `activeUsers` to `total_users` and `active_users` across frontend and backend for consistency.
- Enhanced demo user creation by randomizing `created_at` dates for realistic charts.
- Expanded demo data to include `is_active` for demo users, improving user status representation.
- Refined admin dashboard statistics to support updated user growth data model.

frontend/src/lib/api/admin.ts

@@ -3,8 +3,8 @@ import type { Options } from './generated/sdk.gen';
 
 export interface UserGrowthData {
     date: string;
-    totalUsers: number;
-    activeUsers: number;
+    total_users: number;
+    active_users: number;
 }
 
 export interface OrgDistributionData {