feat(device): 无障碍保护存活心跳检测 + 掉线后置检测(pull)

- device 表 + 心跳/注册/liveness 查询/ack 端点 + heartbeat_monitor_worker
  (扫描超时设备置 kill_alert_pending, 客户端进 App 拉取弹自启动引导)
- 本期只做 pull 后置检测, 不接推送(不含 jpush)
- 迁移: device_table → device_kill_alert_pending(接 main head ceb286289426, 线性单 head)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/models/__init__.py

@@ -12,6 +12,7 @@ from app.models.cps_link import CpsClick, CpsLink  # noqa: F401
 from app.models.cps_order import CpsOrder  # noqa: F401
 from app.models.cps_wx_user import CpsWxUser  # noqa: F401
 from app.models.comparison_milestone import ComparisonMilestoneClaim  # noqa: F401
+from app.models.device import Device  # noqa: F401
 from app.models.coupon_state import (  # noqa: F401
     CouponClaimRecord,
     CouponDailyCompletion,