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.Application/EmailTemplates/Queries/Read/ReadEmailTemplateResponse.cs

@@ -8,17 +8,17 @@ public class ReadEmailTemplateResponse
     /// <summary>
     /// Die eindeutige Kennung der E-Mail-Vorlage.
     /// </summary>
-    public int Id { get; init; }
+    public int Id { get; set; }
 
     /// <summary>
-    /// Der Typ der E-Mail-Vorlage.
+    /// Name des Typs
     /// </summary>
-    public int Type { get; init; }
+    public required string Name { get; set; }
 
     /// <summary>
     /// Das Datum und die Uhrzeit, wann die Vorlage hinzugefügt wurde.
     /// </summary>
-    public DateTime AddedWhen { get; init; }
+    public DateTime AddedWhen { get; set; }
 
     /// <summary>
     /// Der Inhalt (Body) der E-Mail-Vorlage. Kann null sein.