feat(seo): static sitemap generation job via Airflow

- Backend builds sitemap.xml from school data at startup (in-memory)
- POST /api/admin/regenerate-sitemap refreshes it after data updates
- New Airflow DAG (sitemap_generate) runs Sundays 05:00 and calls the endpoint
- Next.js proxies /sitemap.xml to the backend; removes the slow dynamic sitemap.ts
- docker-compose passes BACKEND_URL + ADMIN_API_KEY to Airflow env

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nextjs-app/next.config.js

@@ -6,11 +6,16 @@ const nextConfig = {
   // API Proxy to FastAPI backend
   async rewrites() {
     const apiUrl = process.env.FASTAPI_URL || 'http://localhost:8000/api';
+    const backendUrl = apiUrl.replace(/\/api$/, '');
     return [
       {
         source: '/api/:path*',
         destination: `${apiUrl}/:path*`,
       },
+      {
+        source: '/sitemap.xml',
+        destination: `${backendUrl}/sitemap.xml`,
+      },
     ];
   },