Attach Org Billing

Attach seat billing to an existing organization.

Orgs are created by promote-business, which is deliberate: the billing relationship is established afterwards rather than at signup. This is that second step.

Two modes, because both are real:

• manual_only sets seats_purchased with no provider involved. Comped orgs, self-hosted deployments, and anyone invoiced outside Stripe live here, and they are a supported configuration rather than a broken one.
• Otherwise a Stripe customer and a quantity-N subscription are created. The subscription may come back incomplete when the customer has no payment method yet; that is expected, not a failure. The response carries the status so the operator knows to send the owner a payment link.

Never lowers seats below what is already assigned: doing so would put the org over its own limit the instant billing was attached.