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.

frontend/src/services/admin.service.ts

@@ -0,0 +1,14 @@
+import { api } from "./api";
+
+export interface AdminStats {
+	users: number;
+	contents: number;
+	categories: number;
+}
+
+export const adminService = {
+	getStats: async (): Promise<AdminStats> => {
+		const response = await api.get("/admin/stats");
+		return response.data;
+	},
+};