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.DAL.Abstract/IUnitOfWork.cs

@@ -2,6 +2,7 @@ namespace HrynCo.NotificationService.DAL.Abstract;
 
 public interface IUnitOfWork
 {
+    Task SaveChangesAsync(CancellationToken cancellationToken = default);
     Task<ITransaction> BeginTransactionAsync(CancellationToken cancellationToken = default);
     ITransaction? GetCurrentTransaction();