Finalize a hotel checkout after payment

Runs payment.confirm for the payment transaction created by POST /v1/payments/hotel-checkout-intent. Public by design — it is the second half of the guest-capable payment-first checkout (critical decision #4), and ownership is enforced by the procedure against the transaction's own userId. Call it after the Stripe PaymentSheet reports success, or immediately when the intent came back with testMode: true (staff simulate path). Send {} when there is no provider transaction id to pass through. The booking row, supplier reservation and confirmation email are all created here, so the response carries the booking reference the confirmation screen shows.