fix 标识任务的设备状态

Src/Dlhis.ScheduledTask/TaskAssignment/Response/DeviceTaskAssignmentResponse.cs

@@ -82,4 +82,17 @@ public class DeviceTaskAssignmentResponse
     /// 部署点
     /// </summary>
     public string DeployPlace { get; set; }
-}
+
+    public int HeartbeatInterval { get; set; }
+    
+    public DateTime LastHeartbeatTime { get; set; }
+
+    public bool IsOffline
+    {
+        get
+        {
+            if (HeartbeatInterval == 0) return false;
+            return LastHeartbeatTime.AddSeconds(HeartbeatInterval) > DateTime.Now;
+        }
+    }
+}