feat: 실시간 알림 시스템 (Web Push + 알림 벨 + 서비스간 알림 연동)

- Phase 1: 모든 서비스 헤더에 알림 벨 UI 추가 (notification-bell.js)
- Phase 2: VAPID Web Push 구독/전송 (push-sw.js, pushSubscription API)
- Phase 3: 내부 알림 API + notifyHelper로 서비스간 알림 연동
  - tksafety: 출입 승인/반려, 안전교육 완료, 방문자 체크인
  - tkpurchase: 일용공 신청, 작업보고서 제출
  - system2-report: 신고 접수/확인/처리완료
- Phase 4: 30일 이상 알림 자동 정리 cron, Redis 캐싱
- CORS에 tkuser/tkpurchase/tksafety 서브도메인 추가
- HTML cache busting 버전 갱신

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

system1-factory/api/models/pushSubscriptionModel.js

@@ -0,0 +1,52 @@
+// models/pushSubscriptionModel.js
+const { getDb } = require('../dbPool');
+
+const pushSubscriptionModel = {
+  async subscribe(userId, subscription) {
+    const db = await getDb();
+    const { endpoint, keys } = subscription;
+    await db.query(
+      `INSERT INTO push_subscriptions (user_id, endpoint, p256dh, auth)
+       VALUES (?, ?, ?, ?)
+       ON DUPLICATE KEY UPDATE user_id = VALUES(user_id), p256dh = VALUES(p256dh), auth = VALUES(auth)`,
+      [userId, endpoint, keys.p256dh, keys.auth]
+    );
+  },
+
+  async unsubscribe(endpoint) {
+    const db = await getDb();
+    await db.query('DELETE FROM push_subscriptions WHERE endpoint = ?', [endpoint]);
+  },
+
+  async getByUserId(userId) {
+    const db = await getDb();
+    const [rows] = await db.query(
+      'SELECT * FROM push_subscriptions WHERE user_id = ?',
+      [userId]
+    );
+    return rows;
+  },
+
+  async getByUserIds(userIds) {
+    if (!userIds || userIds.length === 0) return [];
+    const db = await getDb();
+    const [rows] = await db.query(
+      'SELECT * FROM push_subscriptions WHERE user_id IN (?)',
+      [userIds]
+    );
+    return rows;
+  },
+
+  async getAll() {
+    const db = await getDb();
+    const [rows] = await db.query('SELECT * FROM push_subscriptions');
+    return rows;
+  },
+
+  async deleteByEndpoint(endpoint) {
+    const db = await getDb();
+    await db.query('DELETE FROM push_subscriptions WHERE endpoint = ?', [endpoint]);
+  }
+};
+
+module.exports = pushSubscriptionModel;