refactor: replace internal UnitOfWork with NotificationUnitOfWork and NotificationBaseRepository

- Consolidate unit of work implementation with NotificationUnitOfWork.
- Refactor repositories to use NotificationBaseRepository for consistency.
- Simplify request handlers by removing IUnitOfWork dependency.
- Update related tests and service registration.

HrynCo.NotificationService.DAL.EF/ServiceCollectionExtensions.cs

@@ -16,10 +16,10 @@ public static class ServiceCollectionExtensions
         services.AddDbContext<NotificationDbContext>(options =>
             options.UseNpgsql(connectionString));
 
-        services.AddScoped<IUnitOfWork, UnitOfWork>();
         services.AddScoped<IEmailTemplateRepository, EmailTemplateRepository>();
         services.AddScoped<IEmailChannelRepository, EmailChannelRepository>();
         services.AddScoped<IEmailChannelUsageRepository, EmailChannelUsageRepository>();
+        services.AddScoped<IUnitOfWork, NotificationUnitOfWork>();
 
         return services;
     }