Payments Provider Swap

Problem/Feature Description

A subscription product currently talks directly to a payment vendor from application code. Support has seen several incidents caused by callers retrying inconsistently and interpreting vendor response codes differently. The business wants to trial a second provider while keeping subscription workflows stable.

Design and sketch a TypeScript boundary that would let subscription code charge customers without depending on provider quirks. Include enough code to show the public API and one provider implementation stub.

Output Specification

Produce payment_design.md, payments.ts, and result.md.

Input Files

=============== FILE: subscription.ts ===============

export async function renewSubscription(user: User, plan: Plan, stripe: any) {
  const response = await stripe.charges.create({
    customer: user.stripeCustomerId,
    amount: plan.priceCents,
    currency: "usd",
    idempotency_key: `${user.id}-${plan.id}`
  })

  if (response.status === "succeeded") return { ok: true, chargeId: response.id }
  if (response.status === "requires_action") return { ok: false, code: "3ds" }
  if (response.failure_code === "card_declined") return { ok: false, code: "card_declined" }
  throw new Error(response.failure_message)
}