feat: add MediatR handlers for template and channel CRUD

- ServiceResult<T>, ServiceError, ServiceErrorCode, Unit, ServiceResultHelper in Services/Core
- RequestHandler<TRequest, TResponse> base class (MediatR-adapted, DoOnHandle pattern)
- EmailTemplate handlers: Create, Update, Delete, Get, GetByService
- EmailChannel handlers: Create, Update, Delete, Get, GetByService

Ref: IT-628

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

HrynCo.NotificationService.Services/EmailChannels/Delete/DeleteEmailChannelHandler.cs

@@ -0,0 +1,35 @@
+using HrynCo.NotificationService.DAL.Abstract;
+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.EmailChannels.Delete;
+
+internal sealed class DeleteEmailChannelHandler
+    : RequestHandler<DeleteEmailChannelCommand, ServiceResult<Unit>>
+{
+    private readonly IEmailChannelRepository _channels;
+
+    public DeleteEmailChannelHandler(
+        IContextualSerilogLogger<DeleteEmailChannelCommand> logger,
+        IUnitOfWork unitOfWork,
+        IEmailChannelRepository channels)
+        : base(logger, unitOfWork)
+    {
+        _channels = channels;
+    }
+
+    protected override async Task<ServiceResult<Unit>> DoOnHandle(
+        DeleteEmailChannelCommand request, CancellationToken cancellationToken)
+    {
+        var channel = await _channels.GetByIdAsync(request.Id, cancellationToken);
+
+        if (channel is null)
+            return Failure<Unit>("Email channel not found.", ServiceErrorCode.NotFound);
+
+        await _channels.DeleteAsync(channel, cancellationToken);
+
+        return Success(Unit.Value);
+    }
+}