feat(notification): add admin rest management and deletion

Add revision-safe service and template APIs, channel mapping, and validated admin controls.

Ref: IT-1601

HrynCo.NotificationService.Services/TemplateDefinitions/SaveNotificationServiceHandler.cs

@@ -0,0 +1,30 @@
+using HrynCo.NotificationService.DAL.Abstract.Repositories;
+using HrynCo.NotificationService.DAL.Abstract.Templates;
+using HrynCo.NotificationService.Services.Core;
+using HrynCo.NotificationService.Services.Logging;
+using static HrynCo.NotificationService.Services.Core.ServiceResultHelper;
+
+namespace HrynCo.NotificationService.Services.TemplateDefinitions;
+
+internal sealed class SaveNotificationServiceHandler(
+    IContextualSerilogLogger<SaveNotificationServiceCommand> logger, INotificationServiceRepository services)
+    : RequestHandler<SaveNotificationServiceCommand, ServiceResult<NotificationServiceDto>>(logger)
+{
+    protected override async Task<ServiceResult<NotificationServiceDto>> DoOnHandle(SaveNotificationServiceCommand request, CancellationToken cancellationToken)
+    {
+        if (!request.IsNew && request.Service.Revision == Guid.Empty)
+            return Failure<NotificationServiceDto>("A nonempty service revision is required for update.", ServiceErrorCode.InvalidRequest);
+        if (!request.IsNew && await services.GetAsync(request.Service.Id, cancellationToken) is null)
+            return Failure<NotificationServiceDto>("Service not found.", ServiceErrorCode.NotFound);
+        try
+        {
+            await services.SaveAsync(request.Service, request.IsNew, cancellationToken);
+            var saved = await services.GetAsync(request.Service.Id, cancellationToken);
+            return Success(NotificationServiceMapping.Map(saved!));
+        }
+        catch (EmailTemplateWriteException error)
+        {
+            return Failure<NotificationServiceDto>(error.Message, error.IsConflict ? ServiceErrorCode.Conflict : ServiceErrorCode.InvalidRequest);
+        }
+    }
+}