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/EfRepository.cs

@@ -18,40 +18,28 @@ internal abstract class EfRepository<TEntity>
     protected async Task AddAsync(TEntity entity, CancellationToken ct = default)
     {
         await DbSet.AddAsync(entity, ct);
-        await DbContext.SaveChangesAsync(ct);
     }
 
     protected async Task AddRangeAsync(IEnumerable<TEntity> entities, CancellationToken ct = default)
     {
         await DbSet.AddRangeAsync(entities, ct);
-        await DbContext.SaveChangesAsync(ct);
     }
 
-    protected async Task UpdateAsync(TEntity entity, CancellationToken ct = default)
+    protected void Update(TEntity entity)
     {
         DbSet.Update(entity);
-        await DbContext.SaveChangesAsync(ct);
     }
 
-    protected async Task DeleteAsync(TEntity entity, CancellationToken ct = default)
+    protected void Delete(TEntity entity)
     {
         DbSet.Remove(entity);
-        await DbContext.SaveChangesAsync(ct);
     }
 
-    protected async Task DeleteRangeAsync(IEnumerable<TEntity> entities, CancellationToken ct = default)
+    protected void DeleteRange(IEnumerable<TEntity> entities)
     {
         DbSet.RemoveRange(entities);
-        await DbContext.SaveChangesAsync(ct);
     }
 
-    protected Task<bool> ExistsAsync(Expression<Func<TEntity, bool>> predicate, CancellationToken ct = default)
-    {
-        return DbSet.AnyAsync(predicate, ct);
-    }
-
-    protected Task SaveAsync(CancellationToken ct = default)
-    {
-        return DbContext.SaveChangesAsync(ct);
-    }
+    protected Task<bool> ExistsAsync(Expression<Func<TEntity, bool>> predicate, CancellationToken ct = default) =>
+        DbSet.AnyAsync(predicate, ct);
 }