Add audit interfaces and fields to domain entities

Introduced interfaces for audit fields (AddedWhen, ChangedWhen, ChangedWho, AddedWho) and updated domain entities to implement them. Adjusted properties for consistency and nullability. Updated MappingProfile to map audit fields between DTOs and entities. Improves auditability and standardization across the domain model.

EnvelopeGenerator.Domain/Entities/EnvelopeReceiverReadOnly.cs

@@ -1,14 +1,13 @@
-using System.ComponentModel.DataAnnotations.Schema;
+using System;
+using System.ComponentModel.DataAnnotations.Schema;
 using System.ComponentModel.DataAnnotations;
+using EnvelopeGenerator.Domain.Interfaces;
 using DigitalData.EmailProfilerDispatcher.Abstraction.Attributes;
-#if NETFRAMEWORK
-using System;
-#endif
 
 namespace EnvelopeGenerator.Domain.Entities
 {
     [Table("TBSIG_ENVELOPE_RECEIVER_READ_ONLY")]
-    public class EnvelopeReceiverReadOnly
+    public class EnvelopeReceiverReadOnly : IHasAddedWhen, IHasChangedWhen, IHasAddedWho
     {
         [Key]
         [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
@@ -42,16 +41,12 @@ namespace EnvelopeGenerator.Domain.Entities
 
         [Column("ADDED_WHEN")]
         [Required]
-        public DateTime
-#if NET
-            ?
-#endif
-            AddedWhen { get; set; }
+        public DateTime AddedWhen { get; set; }
 
         [Column("CHANGED_WHO")]
         [StringLength(100)]
         public string
-#if NET
+#if nullable
             ?
 #endif
             ChangedWho { get; set; }