Add and verify a custom domain on a Railway service via the API

Documented steps

1. Check availability: query customDomainAvailable(domain)
2. Create it: mutation customDomainCreate with { projectId, environmentId, serviceId, domain, targetPort (optional) }
3. Read status.dnsRecords from the response for the routing records to create, and status.verificationToken for the TXT verification record
4. Create the DNS records at your DNS provider, then poll query customDomain until it verifies
5. Manage later with customDomainUpdate / customDomainDelete; list everything with query domains
6. Docs: https://docs.railway.com/integrations/api/manage-domains