fix(user,customer): return 409 on phone duplicate-key races

Map E11000 from the per-collection phone index to friendly conflict
errors and skip redundant GetByPhone when the phone is unchanged.

Not blocking: IAT test DB may need a one-off dedupe for index build;
phone uniqueness is per collection (users vs customers), not global.

internal/customer/service.go

@@ -156,6 +156,9 @@ func (s *customerService) Register(ctx context.Context, form *CreateCustomerForm
 	// Save to database
 	err = s.repository.Register(ctx, customer)
 	if err != nil {
+		if mapped := mapCustomerWriteError(err); mapped != err {
+			return nil, mapped
+		}
 		s.logger.Error("Failed to create customer", map[string]interface{}{
 			"error":    err.Error(),
 			"email":    form.Email,
@@ -424,6 +427,9 @@ func (s *customerService) Update(ctx context.Context, id string, form *UpdateCus
 	// Update in database
 	err = s.repository.Update(ctx, customer)
 	if err != nil {
+		if mapped := mapCustomerWriteError(err); mapped != err {
+			return nil, mapped
+		}
 		s.logger.Error("Failed to update customer", map[string]interface{}{
 			"error":       err.Error(),
 			"customer_id": id,