Add StatusChangedWhen to DocumentStatus entity

Added a nullable DateTime property, StatusChangedWhen, to the DocumentStatus entity. This property is required and mapped to the "STATUS_CHANGED_WHEN" column, enabling tracking of when the document status was last updated.

EnvelopeGenerator.Domain/Entities/DocumentStatus.cs

@@ -35,6 +35,10 @@ namespace EnvelopeGenerator.Domain.Entities
         [Column("STATUS")]
         public Constants.DocumentStatus Status { get; set; }
 
+        [Required]
+        [Column("STATUS_CHANGED_WHEN", TypeName = "datetime")]
+        public DateTime? StatusChangedWhen { get; set; }
+
         [Required]
         [Column("ADDED_WHEN", TypeName = "datetime")]
         public DateTime AddedWhen { get; set; }