Check if there is a pending offramp that needs USDC sent to Coinbase

Check if the authenticated user has a pending offramp withdrawal where Coinbase has provided a deposit address (to_address). If so, the frontend should send USDC from the user's Smart Account earnings wallet to that address.

Polling pattern: Frontend polls this every ~5 seconds after the user returns from the Coinbase offramp UI.

If a pending event exists but Coinbase hasn't provided a deposit address yet, this endpoint triggers a background Coinbase scan and returns pending=True with to_address=null. The next poll will pick up the address after the scan.