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.EF/Core/EfUnitOfWork.cs

@@ -15,6 +15,11 @@ internal abstract class EfUnitOfWork<TDbContext> : IUnitOfWork
         _context = context;
     }
 
+    public Task SaveChangesAsync(CancellationToken cancellationToken = default)
+    {
+        return _context.SaveChangesAsync(cancellationToken);
+    }
+
     public async Task<ITransaction> BeginTransactionAsync(CancellationToken cancellationToken = default)
     {
         if (_currentTransaction != null)