refactor(device): device 表→device_liveness + liveness 接口只返 bool(review 意见)

- 表/类改名 device→device_liveness / Device→DeviceLiveness:该表记的是无障碍存活监控状态
  (心跳/liveness_state/kill_alert_pending/推送目标),非设备信息(品牌/型号),原名误导。同步改
  建表迁移表名/索引/约束(未部署→直接改建表迁移,非加 rename);API 路径 /api/v1/device 不变。
- GET /device/liveness 的 LivenessOut 只返 kill_alert_pending 一个布尔(原返回 liveness_state/
  ever_protected/last_heartbeat_at 等设备详情);前端本就只读 kill_alert_pending,不受影响。

app/models/__init__.py

@@ -12,7 +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.device import DeviceLiveness  # noqa: F401
 from app.models.coupon_state import (  # noqa: F401
     CouponClaimRecord,
     CouponDailyCompletion,