fix: TiingoTicker CreatedAt/UpdatedAt 改为可空

AssetManager.Data/TiingoTicker.cs

@@ -9,7 +9,7 @@ namespace AssetManager.Data;
 public class TiingoTicker
 {
     /// <summary>
-    /// 主键 ID
+    /// 主键 ID（自增，导入时忽略此字段）
     /// </summary>
     [SugarColumn(IsPrimaryKey = true, IsIdentity = true)]
     public int Id { get; set; }
@@ -51,12 +51,14 @@ public class TiingoTicker
     public DateTime? EndDate { get; set; }
 
     /// <summary>
-    /// 创建时间
+    /// 创建时间（导入时可空，数据库会自动设置）
     /// </summary>
-    public DateTime CreatedAt { get; set; } = DateTime.Now;
+    [SugarColumn(IsNullable = true)]
+    public DateTime? CreatedAt { get; set; }
 
     /// <summary>
-    /// 更新时间
+    /// 更新时间（导入时可空，数据库会自动设置）
     /// </summary>
-    public DateTime UpdatedAt { get; set; } = DateTime.Now;
+    [SugarColumn(IsNullable = true)]
+    public DateTime? UpdatedAt { get; set; }
 }