Records a payment your platform collected from the customer. The
payment is stored as a "system" payment-method type unique to your
platform (auto-created on first use); it never appears in the
merchant's POS settle screen but does appear on the order's payment
history.

Caveats

• Pre-bill rejection. If the order has no bill yet (the merchant
  hasn't accepted it), the endpoint returns 400 with the message
  "Order has not been accepted yet. Payments can only be recorded
  once the merchant accepts the order." Wait for status to advance
  past pending (poll GET /orders/{id} or rely on a webhook) before
  retrying.
• Partial payments are supported. Submit the endpoint multiple
  times with smaller amounts; paid flips to true once the
  cumulative amount covers the bill's outstanding total.
• If your amount exceeds the bill's total_outstanding, you get a
  400 — the system won't accept overpayment.
• Each payment is independent; sending the same ref twice creates
  two payments. Idempotency is your responsibility on the platform
  side.
• ref is truncated to 150 chars.