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/contract/customer_auth_contract.go

@@ -16,6 +16,9 @@ type RegisterStartRequest struct {
 	PhoneNumber string `json:"phone_number" binding:"required"`
 	Name        string `json:"name" binding:"required"`
 	BirthDate   string `json:"birth_date" binding:"required"`
+	// The organization (brand) the customer registers with. A customer belongs to one
+	// organization; the app sends the one it is built for.
+	OrganizationID string `json:"organization_id" binding:"required"`
 }
 
 type RegisterVerifyOtpRequest struct {