Rename ChangedWhen to ActionDate; add wrapper for compatibility

Renamed the ChangedWhen property to ActionDate in the History entity, preserving the database mapping. Added a ChangedWhen property as a wrapper around ActionDate to maintain backward compatibility with existing code.

EnvelopeGenerator.Domain/Entities/History.cs

@@ -35,8 +35,10 @@ namespace EnvelopeGenerator.Domain.Entities
         public DateTime AddedWhen { get; set; }
 
         [Column("ACTION_DATE", TypeName = "datetime")]
-        public DateTime? ChangedWhen { get; set; }
-        
+        public DateTime? ActionDate { get; set; }
+
+        public DateTime? ChangedWhen { get => ActionDate; set => ActionDate = value; }
+
         [Column("COMMENT", TypeName = "nvarchar(max)")]
         public string
 #if nullable