No `throw new Error("not implemented")` in production code

A throw new Error('not implemented') turns into an HTTP 500 the first time any caller hits that code path. Because AI generates these as scaffolding — calculateRefund, sendInvoice, getUserBalance — the thrown paths often sit directly on revenue-critical flows. Users encounter them during checkout, refund, or billing and abandon the transaction; the defect then masquerades as a generic 500 in observability and takes hours to trace back to the stub.

Implement the function against real logic or remove it and adjust every caller. Do not leave scaffold throws behind; a // TODO next to a thrown error is still a 500 in production. Fix at src/lib/billing.ts:

export function calculateRefund(amount: number) {
  const feePercent = 0.03
  return amount * (1 - feePercent)
}