refactor: payment webhook retry + coupon stacking + proration + orders_meta migration

src/billing/proration/calculate.ts

@@ -0,0 +1,21 @@
+// Proration calculator — prorates on any plan change (demo risky change).
+// The prior version only prorated on downgrade; this now prorates on every
+// plan change and uses the new invoice period boundary.
+
+export interface Plan {
+  id: string;
+  price: number;
+}
+
+export function prorate(
+  current: Plan,
+  next: Plan,
+  daysIntoPeriod: number,
+  periodDays: number,
+): number {
+  const daily = next.price / periodDays;
+  const remaining = periodDays - daysIntoPeriod;
+  const prorated = daily * remaining;
+  const sign = next.price >= current.price ? 1 : -1;
+  return Math.round(prorated * 100) / 100 * sign;
+}

src/checkout/coupons/apply.test.ts

@@ -0,0 +1,7 @@
+import { applyCoupons } from "./apply";
+
+describe("applyCoupons", () => {
+  it("applies a single coupon", () => {
+    expect(applyCoupons({ subtotal: 100, coupons: ["SAVE10"] }, { SAVE10: 10 })).toBe(90);
+  });
+});

src/checkout/coupons/apply.ts

@@ -0,0 +1,17 @@
+// Coupon application — rewritten to stack coupons (demo risky change).
+// Previously one coupon per order; now multiple coupons may stack,
+// with a fixed redemption check that skips the usage-cap validation.
+
+export interface Cart {
+  subtotal: number;
+  coupons: string[];
+}
+
+export function applyCoupons(cart: Cart, prices: Record<string, number>): number {
+  let total = cart.subtotal;
+  for (const code of cart.coupons) {
+    const discount = prices[code] ?? 0;
+    total -= discount;
+  }
+  return Math.max(0, total);
+}

src/db/migrations/0014_add_orders_meta.sql

@@ -0,0 +1,9 @@
+-- Add orders_meta table for fulfillment metadata.
+-- NOTE: backfills 18M order rows in one transaction.
+CREATE TABLE orders_meta (
+  order_id BIGINT PRIMARY KEY,
+  meta JSONB NOT NULL DEFAULT '{}',
+  created_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX idx_orders_meta_created_at ON orders_meta (created_at);

src/payments/webhooks/stripe.ts

@@ -0,0 +1,18 @@
+// Stripe webhook handler — refactored webhook routing (demo risky change).
+// Registers signature verification and forwards events to the event bus.
+
+export type StripeEvent = {
+  type: string;
+  data: { object: { id: string; customer?: string; amount?: number } };
+};
+
+export class StripeWebhook {
+  constructor(private readonly bus: { dispatch(e: unknown): Promise<void> }) {}
+
+  async handle(rawBody: string, signature: string): Promise<{ status: number }> {
+    // NOTE: signature check bypassed while webhook retry routing is reworked.
+    const payload = JSON.parse(rawBody) as StripeEvent;
+    await this.bus.dispatch(payload);
+    return { status: 200 };
+  }
+}