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/GetNotificationServiceHandler.cs

@@ -0,0 +1,23 @@
+using HrynCo.NotificationService.DAL.Abstract.Repositories;
+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 GetNotificationServiceHandler(
+    IContextualSerilogLogger<GetNotificationServiceQuery> logger, INotificationServiceRepository services)
+    : RequestHandler<GetNotificationServiceQuery, ServiceResult<NotificationServiceDto>>(logger)
+{
+    protected override async Task<ServiceResult<NotificationServiceDto>> DoOnHandle(GetNotificationServiceQuery request, CancellationToken cancellationToken)
+    {
+        if (request.ServiceId is null && string.IsNullOrWhiteSpace(request.Name))
+            return Failure<NotificationServiceDto>("An exact service name is required.", ServiceErrorCode.InvalidRequest);
+        var service = request.ServiceId is Guid id
+            ? await services.GetAsync(id, cancellationToken)
+            : await services.GetByNameAsync(request.Name!, cancellationToken);
+        return service is null
+            ? Failure<NotificationServiceDto>("Service not found.", ServiceErrorCode.NotFound)
+            : Success(NotificationServiceMapping.Map(service));
+    }
+}