Verify on-chain that a transaction is the USDC payment you expected. Pass ?tx=0x… plus optional ?expectedPayTo=0x… and ?expectedAmount=0.01 (USD); ?network defaults to the deployment chain (base or base-sepolia). Decodes the tx's USDC Transfer logs and returns {verified, actualPayTo, actualAmount, asset, blockTime, confirmations, mismatch}. Unknown/unmined tx → 503 (unbilled). Add ?attest=1 for a signed receipt.