Refactor DTOs to classes with enhanced properties

Changed `EnvelopeReceiverReadOnlyDto` and `EnvelopeReceiverReadOnlyUpdateDto` from records to classes, converting properties to standard get/set accessors and adding documentation comments. Introduced a default value for `ChangedWhen` in `EnvelopeReceiverReadOnlyUpdateDto`.

Modified `ReceiverCreateDto` to a class structure, updating properties to use init-only accessors and encapsulating SHA256 hash logic in a private field. Retained the `AddedWhen` property with its default value.

EnvelopeGenerator.Application/DTOs/EnvelopeReceiverReadOnly/EnvelopeReceiverReadOnlyUpdateDto.cs

@@ -1,14 +1,31 @@
 using DigitalData.Core.Abstractions;
 using Microsoft.AspNetCore.Mvc;
 
-namespace EnvelopeGenerator.Application.DTOs.EnvelopeReceiverReadOnly
+namespace EnvelopeGenerator.Application.DTOs.EnvelopeReceiverReadOnly;
+
+/// <summary>
+/// Data Transfer Object for updating a read-only envelope receiver.
+/// </summary>
+[ApiExplorerSettings(IgnoreApi = true)]
+public class EnvelopeReceiverReadOnlyUpdateDto : IUnique<long>
 {
-    [ApiExplorerSettings(IgnoreApi = true)]
-    public record EnvelopeReceiverReadOnlyUpdateDto(
-        long Id,
-        DateTime DateValid,
-        string ChangedWho) : IUnique<long>
-    {
-        public DateTime ChangedWhen { get; } = DateTime.Now;
-    };
+    /// <summary>
+    /// Gets or sets the unique identifier of the envelope receiver.
+    /// </summary>
+    public long Id { get; init; }
+
+    /// <summary>
+    /// Gets or sets the date when the envelope receiver becomes valid.
+    /// </summary>
+    public DateTime DateValid { get; set; }
+
+    /// <summary>
+    /// Gets or sets the name of the user who made the change.
+    /// </summary>
+    public string ChangedWho { get; set; }
+
+    /// <summary>
+    /// Gets or sets the date and time when the change was made.
+    /// </summary>
+    public DateTime ChangedWhen { get; set; } = DateTime.Now;
 }