Create payment intent

Validates the cart and creates a payment intent with the payment processor. Returns an atlas_pay_url — redirect the diner there to complete payment.

If a pending intent already exists for the same amount, it is returned instead of creating a duplicate.

After payment: Once the diner completes payment on the hosted page, they are redirected back to your application. At that point, call POST /cart/order to validate the payment and convert the cart to an order. Do NOT wait for webhooks — POST /cart/order checks payment status in real-time.