feat: add POST /api/cart/promo endpoint for promo-code discount

[samkerr4coding]

Summary

Adds POST /api/cart/promo — a stateless endpoint that accepts a cart and a promo code and returns the full CartTotals breakdown (subtotal, discount, tax, total) plus a promoApplied boolean.

Re-uses the existing otalize() and �pplyDiscount() functions from lib/cart.ts; no new business logic or DB queries.

Changes

File	Change
�pp/api/cart/promo/route.ts	New POST handler; Zod input validation
ests/promo.test.ts	14 tests: �pplyDiscount branches + route handler
itest.config.ts	Add @/ path alias so vitest can resolve Next.js imports

Request / Response

\
POST /api/cart/promo
Content-Type: application/json

{
"items": [{ "productId": "p1", "quantity": 2, "unitPriceCents": 1000 }],
"promoCode": "WELCOME10",
"region": "GB"
}

HTTP 200
{
"totals": { "subtotalCents": 2000, "discountCents": 200, "taxCents": 360, "totalCents": 2160 },
"promoCode": "WELCOME10",
"promoApplied": true
}
\\

Checks

• 
  pm run lint\ — no warnings or errors
• 
  pm test\ — 21/21 tests pass (14 new)

Out of scope — noted for team

The following were intentionally excluded from this work item; a human should decide on priority:

1. Promo code persistence — codes are hardcoded in \�pplyDiscount(). A \promo_codes\ table + admin CRUD would be needed for dynamic codes.
2. Redemption tracking — no per-user usage limits or expiry enforcement.
3. Auth on this endpoint — consistent with the existing /api/products\ route (no auth), since this is a read-only calculation. Team should confirm whether a session cookie or Bearer token is required here before GA.