feat(ntfy): Phase 2 — sendPushToUsers() ntfy 연동 + 구독 관리 UI

- ntfy_subscriptions 테이블 마이그레이션 추가
- ntfySender.js 유틸 (ntfy HTTP POST 래퍼)
- sendPushToUsers() ntfy 우선 분기 (ntfy 구독자 → ntfy, 나머지 → Web Push)
- ntfy subscribe/unsubscribe/status API 엔드포인트
- notification-bell.js ntfy 토글 버튼 + 앱 설정 안내 모달
- docker-compose system1-api에 NTFY 환경변수 추가

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

docker-compose.yml

@@ -107,6 +107,10 @@ services:
       - VAPID_PRIVATE_KEY=${VAPID_PRIVATE_KEY}
       - VAPID_SUBJECT=${VAPID_SUBJECT:-mailto:admin@technicalkorea.net}
       - INTERNAL_SERVICE_KEY=${INTERNAL_SERVICE_KEY}
+      - NTFY_BASE_URL=${NTFY_BASE_URL:-http://ntfy:80}
+      - NTFY_PUBLISH_TOKEN=${NTFY_PUBLISH_TOKEN}
+      - NTFY_EXTERNAL_URL=${NTFY_EXTERNAL_URL:-https://ntfy.technicalkorea.net}
+      - TKFB_BASE_URL=${TKFB_BASE_URL:-https://tkfb.technicalkorea.net}
     volumes:
       - system1_uploads:/usr/src/app/uploads
       - system1_logs:/usr/src/app/logs