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/ElementAnnotation.cs

@@ -1,18 +1,19 @@
-using System.ComponentModel.DataAnnotations;
+using System;
+using EnvelopeGenerator.Domain.Interfaces;
+using System.ComponentModel.DataAnnotations;
 using System.ComponentModel.DataAnnotations.Schema;
 #if NETFRAMEWORK
-using System;
 #endif
 
 namespace EnvelopeGenerator.Domain.Entities
 #if NET
     ;
 #elif NETFRAMEWORK
-    {
+{
 #endif
 
 [Table("TBSIG_DOCUMENT_RECEIVER_ELEMENT_ANNOTATION")]
-public class ElementAnnotation
+public class ElementAnnotation : IHasAddedWhen, IHasChangedWhen, IHasChangedWho
 {
     [Key]
     [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
@@ -74,14 +75,14 @@ public class ElementAnnotation
     [Column("CHANGED_WHO", TypeName = "nvarchar(100)")]
     [StringLength(100)]
     public string
-#if NET
+#if nullable
         ?
 #endif
         ChangedWho { get; set; }
 
     [ForeignKey("ElementId")]
     public virtual Signature
-#if NET
+#if nullable
         ?
 #endif
         Element { get; set; }