feat: Web Push 비활성 운영 설정을 준비한다

VAPID private key를 Compose secret으로 주입하고 Web Push를 기본 비활성으로 전달한다. 활성화 입력과 Compose 모델 검증, 운영 README를 함께 추가한다.

compose.yaml

@@ -117,6 +117,12 @@ services:
       GATEWAY_ADMIN_LOCAL_ACCOUNT_ENABLED: ${GATEWAY_ADMIN_LOCAL_ACCOUNT_ENABLED:-true}
       GATEWAY_LOCAL_ACCOUNT_GRACE_DAYS: ${GATEWAY_LOCAL_ACCOUNT_GRACE_DAYS:-7}
       GATEWAY_LEGACY_PASSWORD_GLOBAL_SALT: ${GATEWAY_LEGACY_PASSWORD_GLOBAL_SALT:-}
+      WEB_PUSH_ENABLED: ${WEB_PUSH_ENABLED:-false}
+      WEB_PUSH_VAPID_SUBJECT: ${WEB_PUSH_VAPID_SUBJECT:-}
+      WEB_PUSH_VAPID_PUBLIC_KEY: ${WEB_PUSH_VAPID_PUBLIC_KEY:-}
+      WEB_PUSH_VAPID_PRIVATE_KEY_FILE: /run/secrets/web_push_vapid_private_key
+      WEB_PUSH_POLL_INTERVAL_MS: ${WEB_PUSH_POLL_INTERVAL_MS:-1000}
+      WEB_PUSH_OUTBOX_POLL_MS: ${WEB_PUSH_OUTBOX_POLL_MS:-1000}
       GATEWAY_API_HOST: 0.0.0.0
       GATEWAY_API_PORT: '15001'
       GATEWAY_FRONTEND_PORT: '15000'
@@ -151,6 +157,8 @@ services:
       - frontend-artifacts:/srv/frontend-artifacts
       - ${IMAGE_UPLOAD_CORE2026_SECRET_FILE:?set IMAGE_UPLOAD_CORE2026_SECRET_FILE in .env}:/run/secrets/image_upload_core2026_secret:ro
       - ${IMAGE_SYNC_CORE2026_SECRET_FILE:?set IMAGE_SYNC_CORE2026_SECRET_FILE in .env}:/run/secrets/image_sync_core2026_secret:ro
+    secrets:
+      - web_push_vapid_private_key
     depends_on:
       postgres:
         condition: service_healthy
@@ -192,6 +200,10 @@ services:
         condition: service_healthy
     restart: unless-stopped
 
+secrets:
+  web_push_vapid_private_key:
+    file: ${WEB_PUSH_VAPID_PRIVATE_KEY_FILE:-./secrets/web_push_vapid_private_key.example}
+
 volumes:
   postgres-data:
   redis-data: