fix(customer-auth): register customers into the app's organization

Registration put every new customer into a hardcoded organization id,
which does not exist in staging, so set-password failed on the
fk_customers_organization foreign key with a 500.

POST /customer-auth/register/start now takes organization_id, the
organization the app is built for. It must be a UUID of an existing
organization, checked before the OTP is sent; it is kept in the OTP
session and set-password creates the customer there. A registration
started before this change has no organization in its session and is asked
to start again.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/repository/customer_auth_repository.go

@@ -6,6 +6,7 @@ import (
 
 	"apskel-pos-be/internal/entities"
 
+	"github.com/google/uuid"
 	"gorm.io/gorm"
 )
 
@@ -16,6 +17,8 @@ type CustomerAuthRepository interface {
 	UpdateCustomer(ctx context.Context, customer *entities.Customer) error
 	CheckPhoneNumberExists(ctx context.Context, phoneNumber string) (bool, error)
 	SetCustomerPassword(ctx context.Context, customerID string, passwordHash string) error
+	// OrganizationExists reports whether an organization with this id exists.
+	OrganizationExists(ctx context.Context, organizationID uuid.UUID) (bool, error)
 }
 
 type customerAuthRepository struct {
@@ -78,3 +81,12 @@ func (r *customerAuthRepository) SetCustomerPassword(ctx context.Context, custom
 	}
 	return nil
 }
+
+func (r *customerAuthRepository) OrganizationExists(ctx context.Context, organizationID uuid.UUID) (bool, error) {
+	var count int64
+	err := r.db.WithContext(ctx).Table("organizations").Where("id = ?", organizationID).Count(&count).Error
+	if err != nil {
+		return false, fmt.Errorf("failed to check organization: %w", err)
+	}
+	return count > 0, nil
+}