feat(api): add promo endpoint to apply percentage discount

app/api/promo/route.ts

@@ -0,0 +1,38 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { z } from 'zod';
+import { cartItemSchema, computeTax, totalize } from '@/lib/cart';
+
+const BodySchema = z.object({
+  cart: z.array(cartItemSchema),
+  // either a direct percent (0-100) or a promo `code` handled by existing logic
+  percent: z.number().int().min(0).max(100).optional(),
+  code: z.string().min(1).optional(),
+  region: z.string().min(2).optional().default('GB'),
+});
+
+export async function POST(req: NextRequest) {
+  const body = await req.json().catch(() => null);
+  const parsed = BodySchema.safeParse(body);
+  if (!parsed.success) {
+    return NextResponse.json({ error: 'invalid_body' }, { status: 400 });
+  }
+
+  const { cart, percent, code, region } = parsed.data;
+
+  const subtotalCents = cart.reduce((s, it) => s + it.unitPriceCents * it.quantity, 0);
+
+  let discountCents = 0;
+  if (typeof percent === 'number') {
+    discountCents = Math.floor((subtotalCents * percent) / 100);
+  } else if (code) {
+    // reuse existing discount logic when a code is provided
+    const totals = totalize(cart, code, region);
+    discountCents = totals.discountCents;
+  }
+
+  const taxableCents = Math.max(0, subtotalCents - discountCents);
+  const taxCents = computeTax(taxableCents, region);
+  const totalCents = taxableCents + taxCents;
+
+  return NextResponse.json({ subtotalCents, discountCents, taxCents, totalCents });
+}