Register Shopify User

Create only the Pomo identity for a verified Shopify-origin install.

The normal registration endpoint deliberately creates a Stripe organization and enters Stripe checkout. A Shopify merchant must never cross that boundary. This endpoint is gated by the browser-bound Shopify claim, records the same account details and Terms acceptance, and leaves organization and profile creation to the atomic Shopify resolve path.

An existing identity whose registration is unfinished (typically a Stripe register/init that never paid) is moved onto the Shopify path; one whose Stripe payment has begun is refused with stripe_registration_billing_started. A completed registration is left untouched. The response reports the resulting registration_type and registration_complete so the client continues to Shopify only on the server's confirmation.