Enhance Notification and User Services with Improved Error Handling and Data Consistency

- Updated GetByID methods in user and customer services to return specific "not found" errors for better clarity in error responses.
- Modified NotificationResponse structure to include a new Recipient field, allowing for more flexible recipient handling.
- Changed CreatedAt and UpdatedAt fields in NotificationResponse to use Unix timestamps (int64) for consistency across the application.
- Improved notification retrieval logic to handle potential nil values for user and customer lookups, ensuring robust data handling.

internal/customer/service.go

@@ -168,6 +168,9 @@ func (s *customerService) Register(ctx context.Context, form *CreateCustomerForm
 func (s *customerService) GetByID(ctx context.Context, id string) (*CustomerResponse, error) {
 	customer, err := s.repository.GetByID(ctx, id)
 	if err != nil {
+		if err.Error() == "customer not found" {
+			return nil, errors.New("customer not found")
+		}
 		s.logger.Error("Failed to get customer by ID", map[string]interface{}{
 			"error":       err.Error(),
 			"customer_id": id,