feat(device): 无障碍掉线后置检测 — kill_alert_pending + liveness 查询/ack 端点

推送暂缓改 pull:worker 检出掉线即置 kill_alert_pending(与 liveness_state 解耦,心跳恢复 touch_heartbeat 不清,规避竞态),客户端进 App 拉 GET /device/liveness 命中→弹自启动引导,POST /device/liveness/ack 清。含 alembic 加列(临时 sqlite 验通)。spec: accessibility-liveness-pull-prompt.md

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

app/api/v1/device.py

@@ -19,6 +19,8 @@ from app.schemas.device import (
     DeviceOut,
     DeviceRegisterRequest,
     HeartbeatRequest,
+    LivenessAckRequest,
+    LivenessOut,
     OkResponse,
 )
 
@@ -64,3 +66,31 @@ def report_heartbeat(
         registration_id=req.registration_id,
     )
     return OkResponse()
+
+
+@router.get("/liveness", response_model=LivenessOut, summary="查询本机掉线告警(后置检测)")
+def get_liveness(
+    device_id: str,
+    user: CurrentUser,
+    db: DbSession,
+) -> LivenessOut:
+    """客户端进 App 时拉取: 本机是否被判定掉线过(kill_alert_pending)。
+
+    设备从未注册过 → 返回默认(无告警)。命中 → 客户端弹「开启自启动」引导, 之后调 /liveness/ack 清。
+    见 spec: spec/accessibility-liveness-pull-prompt.md。
+    """
+    device = device_repo.get_device(db, user_id=user.id, device_id=device_id)
+    if device is None:
+        return LivenessOut()
+    return LivenessOut.model_validate(device)
+
+
+@router.post("/liveness/ack", response_model=OkResponse, summary="确认已提醒(清掉线告警)")
+def ack_liveness(
+    req: LivenessAckRequest,
+    user: CurrentUser,
+    db: DbSession,
+) -> OkResponse:
+    """客户端已弹过「开启自启动」引导 → 清掉「待提醒」标记(下次真掉线 worker 再置)。"""
+    device_repo.ack_kill_alert(db, user_id=user.id, device_id=req.device_id)
+    return OkResponse()