Recurring USDC billing that never holds the money.

// 1. The customer signs once. Retainer registers it on-chain and pays the gas.
const permission = await requestSpendPermission({
  account,
  spender: RETAINER_ROUTER,                 // 0x3370…cC79, verified on Basescan
  token: USDC,
  allowance: 20_000_000n,                   // 20 USDC per period — a ceiling, not a price
  periodInDays: 30,                         // resets on-chain; nothing carries over
  extraData: encodeExtraData(executor, merchantTreasury),
  provider,
});

// 2. Charge whatever this period actually cost, up to the cap, when it's due.
await enqueueCharge({ permission, amount: usageThisPeriod });

// 3. "Paid" means the on-chain events say so — never that a transaction was sent.