New Unpaid Tenant

Buying a workspace must always be possible, also after the free trial was used (P0): this creates (or returns the caller's existing) workspace on plan=unpaid — READ-ONLY until its first paid subscription (tenant_access awaiting_payment; routes/billing._fulfil_subscription sets plan=paid). It grants nothing for free and does not consume or start a trial; the buyer then subscribes it through the normal /billing/checkout (target=tenant, plan=subscribe).