first commit

backend/F9WebAiTest.Domain/Common/IEntity.cs

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+namespace F9WebAiTest.Domain.Common;
+
+/// <summary>实体公共字段接口（Id/CreatedAt/UpdatedAt/IsDeleted），用于统一新旧两套实体基类</summary>
+public interface IEntity
+{
+    long Id { get; set; }
+    DateTime CreatedAt { get; set; }
+    DateTime? UpdatedAt { get; set; }
+    bool IsDeleted { get; set; }
+}