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.module.ts

@@ -0,0 +1,14 @@
+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 {}