Pay a Scout credits top-up intent

x402 resource that settles one Scout credits top-up intent created by POST /api/account/scout-credits/intent. No session is required or accepted here: the intent already bound topup_id to an account, which is what lets a stock x402 client with an unauthenticated discovery request (awal x402 pay <pay_url>, for example) pay it. Every request is gated on the intent first, before any payment work: an unknown topup_id is 404; an already-settled or expired one is 409. Idempotent on settlement: a second successful payment against an already-settled topup_id is never credited twice.