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>

docker-compose.yml

@@ -119,6 +119,8 @@ services:
       PG_DATABASE: schoolcompare
       TYPESENSE_URL: http://typesense:8108
       TYPESENSE_API_KEY: ${TYPESENSE_API_KEY:-changeme}
+      BACKEND_URL: http://backend:80
+      ADMIN_API_KEY: ${ADMIN_API_KEY:-changeme}
     volumes:
 
     depends_on: