Notify a paid order

Closes the source cart, emits order_paid, placed_order and one ordered_product per item, and records the conversion. A buyer who is not a contact yet is created as a transactional-only contact (consent not_subscribed: receipts and notices, no marketing campaigns or automations, and not counted toward the plan's contact limit) instead of dropping the order. The events are deduplicated on externalId: sending the same order again returns 202 without emitting second events or starting post-purchase automations twice. Required scope: commerce:write. Requires a plan with the ecommerce feature.