Queue a backend-paid native USDC transfer for the ARC mainnet faucet.

Arc's native token is USDC, so this is a plain native transfer with the amount encoded at 18 decimals (one claim pays 0.1 USDC). Users do not need gas: the record is inserted into point_transactions and the reward payer service broadcasts the transfer. A dedicated reserved user_nonce keeps ARC claims independent from GIWA faucet claims while the (user, user_nonce) unique index still blocks duplicates.