Create a Sub-merchant

Send POST /onboarding/v1/sub-merchants with merchantId in the request header. The current JSON request supports subMerchantType (default DEFAULT), subMerchantExternalId, name, address, email, iban, gsmNumber, website, companyEmail, zipCode, contactPhoneNumber, cityId, districtId, monthlyIncome, taxOffice, taxNumber, contactName, contactSurname, identityNumber, and legalCompanyTitle, subject to the DTO's validation rules.

A successful request returns 201 Created with id and status. Registration and document-based onboarding flows are also exposed at /register and /createWithFile; the latter uses multipart form data.