refactor: rename domain types and introduce TransactionBehavior pattern

- Rename Template -> EmailTemplate, Provider -> EmailChannel,
  ProviderSettings -> EmailChannelSettings, ProviderType -> EmailChannelType,
  ProviderUsage -> EmailChannelUsage throughout all layers
- Add Undefined = 0 to EmailChannelType enum for safe default handling
- Remove SaveChangesAsync from EfRepository methods — repositories now only stage changes
- Add SaveChangesAsync to IUnitOfWork and EfUnitOfWork
- Add TransactionBehavior MediatR pipeline: wraps every handler in a transaction,
  saves and commits on success, rolls back on exception
- Add MediatR package reference to Services project

Ref: IT-628

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

HrynCo.NotificationService.Services/Behaviors/TransactionBehavior.cs

@@ -0,0 +1,25 @@
+using HrynCo.NotificationService.DAL.Abstract;
+using MediatR;
+
+namespace HrynCo.NotificationService.Services.Behaviors;
+
+public class TransactionBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
+    where TRequest : notnull
+{
+    private readonly IUnitOfWork _unitOfWork;
+
+    public TransactionBehavior(IUnitOfWork unitOfWork)
+    {
+        _unitOfWork = unitOfWork;
+    }
+
+    public Task<TResponse> Handle(TRequest request, RequestHandlerDelegate<TResponse> next, CancellationToken cancellationToken)
+    {
+        return _unitOfWork.ExecuteInTransactionAsync(async () =>
+        {
+            TResponse result = await next();
+            await _unitOfWork.SaveChangesAsync(cancellationToken);
+            return result;
+        });
+    }
+}