Authorization payment flow

1. The client requests a protected resource.
2. The server returns HTTP 402 with PAYMENT-REQUIRED.
3. The client selects an allowed requirement, signs one exact XRPL Payment, and retries once with PAYMENT-SIGNATURE if the body is replayable.
4. The resource server asks the facilitator to verify the payload.
5. If valid, the protected handler runs. Its complete response is buffered.
6. Only a successful 2xx/3xx handler response is submitted for settlement.
7. The facilitator reserves the transaction hash, submits once, and waits for a validated ledger result.
8. The server releases the buffered response with PAYMENT-RESPONSE only after settlement succeeds.

Invalid payments never run the handler. Handler errors and 4xx/5xx responses are never settled. Pending settlement retries reuse the same envelope; an unsafe handler is recovered from Redis rather than executed twice.