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>

system1-factory/api/routes/pushSubscriptionRoutes.js

@@ -11,4 +11,9 @@ router.get('/vapid-public-key', pushController.getVapidPublicKey);
 router.post('/subscribe', requireAuth, pushController.subscribe);
 router.delete('/unsubscribe', requireAuth, pushController.unsubscribe);
 
+// ntfy 구독 관리
+router.post('/ntfy/subscribe', requireAuth, pushController.ntfySubscribe);
+router.delete('/ntfy/unsubscribe', requireAuth, pushController.ntfyUnsubscribe);
+router.get('/ntfy/status', requireAuth, pushController.ntfyStatus);
+
 module.exports = router;