Refactor UpdateEmailTemplateCommand and handler

- Introduce EmailTemplateUpdateDto for update payloads.
- Expose Id and Type directly on UpdateEmailTemplateCommand; remove EmailTemplateQuery property.
- Add QueryExpression for flexible template selection by Id or Type.
- Remove AutoMapper and EmailTemplateDto usage from handler; update repository call to use EmailTemplateUpdateDto.
- Update MappingProfile to map EmailTemplateUpdateDto to EmailTemplate and set ChangedWhen to DateTime.UtcNow.
- Remove obsolete code and improve documentation.

EnvelopeGenerator.Application/EmailTemplates/MappingProfile.cs

@@ -17,8 +17,7 @@ public class MappingProfile : Profile
     {
         CreateMap<EmailTemplate, EmailTemplateDto>();
 
-        CreateMap<UpdateEmailTemplateCommand, EmailTemplate>()
-            .ForMember(dest => dest.Id, opt => opt.Ignore())
-            .ForMember(dest => dest.ChangedWhen, opt => opt.MapFrom(_ => DateTime.Now));
+        CreateMap<EmailTemplateUpdateDto, EmailTemplate>()
+            .ForMember(dest => dest.ChangedWhen, opt => opt.MapFrom(_ => DateTime.UtcNow));
     }
 }