feat(admin): implement admin statistics API and service

Add admin statistics endpoint to provide user, content, and category stats. Introduce `AdminModule` with controller, service, and repository integration for data aggregation. Include frontend service to consume the stats API.

backend/src/admin/admin.controller.ts

@@ -0,0 +1,17 @@
+import { Controller, Get, UseGuards } from "@nestjs/common";
+import { AuthGuard } from "../auth/guards/auth.guard";
+import { RolesGuard } from "../auth/guards/roles.guard";
+import { Roles } from "../auth/decorators/roles.decorator";
+import { AdminService } from "./admin.service";
+
+@Controller("admin")
+@UseGuards(AuthGuard, RolesGuard)
+@Roles("admin")
+export class AdminController {
+	constructor(private readonly adminService: AdminService) {}
+
+	@Get("stats")
+	getStats() {
+		return this.adminService.getStats();
+	}
+}