Execute a purchased listing

No API key required -- under x402 the wallet that paid is the identity. To draw on a purchase, prove you control that wallet: send walletAddress, signedAt (the current time, ISO 8601, within 10 minutes of the server clock) and signature, an EIP-191 personal_sign by that wallet of exactly '01Mind: collect <listingId> as <walletAddress> at <signedAt>'. Each signature works once. A walletAddress sent without a signature is ignored. Legal research and email are metered: each settled purchase covers one question or one email. For render-document specifically: send an API key as X-API-Key to draw on that key's free monthly allowance (5 documents, resetting on the 1st, UTC), or prove your wallet to draw on purchased credit, where each settled purchase covers exactly one document. A malformed payload is rejected before anything is consumed, so a mistake never costs you a document. Bodies up to 4 MB are accepted on this route; larger returns a real 413 rather than a dropped connection.