Enhance email template handling and documentation

- Added XML documentation to the `Handle` method in `UpdateEmailTemplateCommandHandler`.
- Improved readability in `ReadEmailTemplateQueryHandler` by storing the mapped response in a variable.
- Updated properties in `ReadEmailTemplateResponse` to be mutable and renamed `Type` to `Name` with a type change from `int` to `string`.
- Added data annotations in `EmailTemplate` for `AddedWhen` and introduced a new nullable `ChangedWhen` property.
- Included necessary using directives for data annotations in `EmailTemplate.cs`.

EnvelopeGenerator.Domain/Entities/EmailTemplate.cs

@@ -1,4 +1,5 @@
 using DigitalData.Core.Abstractions;
+using System.ComponentModel;
 using System.ComponentModel.DataAnnotations;
 using System.ComponentModel.DataAnnotations.Schema;
 
@@ -20,5 +21,13 @@ namespace EnvelopeGenerator.Domain.Entities
 
         [Column("SUBJECT", TypeName = "nvarchar(512)")]
         public string? Subject { get; set; }
+
+        [Required]
+        [Column("ADDED_WHEN", TypeName = "datetime")]
+        [DefaultValue("GETDATE()")]
+        public DateTime AddedWhen { get; set; }
+
+        [Column("CHANGED_WHEN", TypeName = "datetime")]
+        public DateTime? ChangedWhen { get; set; }
     }
 }