统一AssignmentStatus字段类型，新建枚举，对应状态

QYZH.InteractiveMagazine.Models/Entity/JournalPageTaskUserAnswerSnapshot.cs

@@ -171,6 +171,6 @@ namespace QYZH.InteractiveMagazine.Models.Entity
         /// <summary>
         /// Assignment status.
         /// </summary>
-        public string? AssignmentStatus { get; set; }
+        public int? AssignmentStatus { get; set; }
     }
 }