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/Interfaces/IHasAddedWhen.cs

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+#if NETFRAMEWORK
+using System;
+#endif
+
+namespace EnvelopeGenerator.Domain.Interfaces
+{
+    public interface IHasAddedWhen
+    {
+        DateTime AddedWhen { get; set; }
+    }
+}

EnvelopeGenerator.Domain/Interfaces/IHasAddedWho.cs

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+#if NETFRAMEWORK
+using System;
+#endif
+
+namespace EnvelopeGenerator.Domain.Interfaces
+{
+    public interface IHasAddedWho
+    {
+        string AddedWho { get; set; }
+    }
+}

EnvelopeGenerator.Domain/Interfaces/IHasChangedWhen.cs

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+#if NETFRAMEWORK
+using System;
+#endif
+
+namespace EnvelopeGenerator.Domain.Interfaces
+{
+    public interface IHasChangedWhen
+    {
+        DateTime? ChangedWhen { get; set; }
+    }
+}

EnvelopeGenerator.Domain/Interfaces/IHasChangedWho.cs

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+namespace EnvelopeGenerator.Domain.Interfaces
+{
+    public interface IHasChangedWho
+    {
+        string
+#if nullable
+            ?
+#endif
+            ChangedWho { get; set; }
+    }
+}