Add mempool-feerate
mempool-feerate is a small paid HTTP endpoint (GET /paid/finance/mempool-feerate, 100 sats via L402). After Lightning payment it returns JSON with three on-chain fee bands — fast / medium / slow — in sats per virtual byte, plus a timestamp and a short cache TTL.
It pulls public estimates from mempool.space’s recommended-fees API, digests them into those three integers, and caches the result for 15–60 seconds. It does not replace mempool.space, scrape the website, or help with Lightning-only invoice pays. It is for an agent that is about to send on-chain bitcoin (sweep, channel open/close, on-chain pay) and needs a current fee hint so it neither underpays nor overpays.