Complete pending native ETH/MON SEND user actions (cron)

Expected schedule: every 4 minutes.

Finds pending (status=0) SEND user actions whose transfer.token is eth or mon, paginates through them, checks on-chain transaction status on the action sourceChainId (e.g. Ethereum/Base for ETH, Monad for MON), and when mined successfully: marks the SEND as completed, updates SEND + transfer info from the transaction/receipt (block number, timestamp, value, from/to), and creates the matching RECEIVE user action. Returns 204 with an empty body on success. Requires X-CRON-API-KEY matching CRON_API_KEY.