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.service.ts

@@ -0,0 +1,27 @@
+import { Injectable } from "@nestjs/common";
+import { UsersRepository } from "../users/repositories/users.repository";
+import { ContentsRepository } from "../contents/repositories/contents.repository";
+import { CategoriesRepository } from "../categories/repositories/categories.repository";
+
+@Injectable()
+export class AdminService {
+	constructor(
+		private readonly usersRepository: UsersRepository,
+		private readonly contentsRepository: ContentsRepository,
+		private readonly categoriesRepository: CategoriesRepository,
+	) {}
+
+	async getStats() {
+		const [userCount, contentCount, categoryCount] = await Promise.all([
+			this.usersRepository.countAll(),
+			this.contentsRepository.count({}),
+			this.categoriesRepository.countAll(),
+		]);
+
+		return {
+			users: userCount,
+			contents: contentCount,
+			categories: categoryCount,
+		};
+	}
+}