refactor(admin): CRM을 apps/admin 독립 프로젝트로 분리

- apps/web에서 admin 라우트/가드/sidebar 링크 제거
- apps/admin: 독립 Next.js 앱 (포트 3001)
  - 자체 login/unauthorized/auth callback
  - admin-sidebar: Overview/Users/Subscriptions/Usage
  - requireAdmin() 가드: profile.role='admin' 체크
- monorepo workspace에 apps/admin 등록

apps/admin/next-env.d.ts

@@ -0,0 +1,6 @@
+/// <reference types="next" />
+/// <reference types="next/image-types/global" />
+/// <reference path="./.next/types/routes.d.ts" />
+
+// NOTE: This file should not be edited
+// see https://nextjs.org/docs/app/api-reference/config/typescript for more information.