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>

pipeline/dags/sitemap_dag.py

@@ -0,0 +1,62 @@
+"""
+Sitemap Generation DAG
+
+Rebuilds the sitemap.xml on the backend after data updates so search engines
+get fresh school URLs. Runs weekly (Sunday 05:00) and can be triggered manually.
+"""
+
+from __future__ import annotations
+
+from datetime import datetime, timedelta
+
+from airflow import DAG
+
+try:
+    from airflow.providers.standard.operators.bash import BashOperator
+except ImportError:
+    from airflow.operators.bash import BashOperator
+
+default_args = {
+    "owner": "school-compare",
+    "depends_on_past": False,
+    "email_on_failure": False,
+    "retries": 2,
+    "retry_delay": timedelta(minutes=5),
+}
+
+with DAG(
+    dag_id="sitemap_generate",
+    default_args=default_args,
+    description="Rebuild sitemap.xml on the backend after school data updates",
+    schedule="0 5 * * 0",  # Sundays 05:00 — after the weekly data window
+    start_date=datetime(2025, 1, 1),
+    catchup=False,
+    tags=["school-compare", "seo"],
+) as dag:
+
+    regenerate_sitemap = BashOperator(
+        task_id="call_regenerate_sitemap",
+        bash_command="""
+set -e
+BACKEND_URL="${BACKEND_URL:-http://backend:80}"
+ADMIN_KEY="${ADMIN_API_KEY:-changeme}"
+
+echo "Calling $BACKEND_URL/api/admin/regenerate-sitemap ..."
+
+response=$(curl -s -o /tmp/sitemap_response.json -w "%{http_code}" \\
+    -X POST "$BACKEND_URL/api/admin/regenerate-sitemap" \\
+    -H "X-API-Key: $ADMIN_KEY" \\
+    -H "Content-Type: application/json")
+
+echo "HTTP status: $response"
+cat /tmp/sitemap_response.json
+
+if [ "$response" != "200" ]; then
+    echo "ERROR: sitemap regeneration failed (HTTP $response)"
+    exit 1
+fi
+
+urls=$(python3 -c "import json,sys; d=json.load(open('/tmp/sitemap_response.json')); print(d.get('urls', '?'))")
+echo "Sitemap regenerated: $urls URLs."
+""",
+    )