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();
+	}
+}

backend/src/admin/admin.module.ts

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+import { Module } from "@nestjs/common";
+import { AuthModule } from "../auth/auth.module";
+import { UsersModule } from "../users/users.module";
+import { ContentsModule } from "../contents/contents.module";
+import { CategoriesModule } from "../categories/categories.module";
+import { AdminController } from "./admin.controller";
+import { AdminService } from "./admin.service";
+
+@Module({
+	imports: [AuthModule, UsersModule, ContentsModule, CategoriesModule],
+	controllers: [AdminController],
+	providers: [AdminService],
+})
+export class AdminModule {}

backend/src/admin/admin.service.ts

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+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,
+		};
+	}
+}

frontend/src/services/admin.service.ts

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+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;
+	},
+};