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/Create/CreateEmailChannelCommand.cs

@@ -0,0 +1,16 @@
+using HrynCo.NotificationService.DAL.Abstract.Providers;
+using MediatR;
+using HrynCo.NotificationService.Services.Core;
+
+namespace HrynCo.NotificationService.Services.EmailChannels.Create;
+
+public sealed record CreateEmailChannelCommand(
+    string ServiceName,
+    int Priority,
+    EmailChannelType ChannelType,
+    EmailChannelSettings Settings,
+    int? DailyLimit,
+    int? MonthlyLimit,
+    int WarnThresholdPercent,
+    bool IsActive
+) : IRequest<ServiceResult<Guid>>;