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.config.mjs

@@ -0,0 +1,10 @@
+/** @type {import('next').NextConfig} */
+const nextConfig = {
+  reactStrictMode: true,
+  transpilePackages: ['@d3ro/core', '@d3ro/ui', '@d3ro/api-client'],
+  experimental: {
+    optimizePackageImports: ['@mui/material', '@mui/icons-material', '@d3ro/ui']
+  }
+}
+
+export default nextConfig