# Architect — F-019 Promotions v1

## Feature
F-019 adds server-side promo code validation and discount calculation. Discounts are recalculated from current cart totals and never trusted from the client.

## Design

### Module boundaries
Create `project/src/modules/promotions/` with domain/application/infrastructure/api/tests. Promotions owns promo definitions and validation. Cart receives a public `PromotionServicePort` from the composition root and stores only the selected promo code, not discount amounts.

### Data model
Add migration `014_promotions.js`:
- `promotions_promotions`: code, type (`percent`, `fixed_amount`), value, validity window, usage_limit, usage_count, active, timestamps.
- Add `promo_code text` to `cart_carts`.

### Rules
- Percent discounts are basis points over current cart total.
- Fixed amount discounts are cents and capped at current cart total.
- Validity checks: active, starts_at <= now, ends_at >= now, usage_count < usage_limit when limit exists.
- No stacking in v1: one promo code per cart.
- Client discount fields are stripped/ignored by cart schemas.

### API
- Admin `POST /promotions` creates promo codes.
- Public/authenticated cart endpoint `POST /cart/promo-code` stores a code after server validation.
- `GET /cart` recalculates discount through `PromotionService`.

## Acceptance trace
- Valid promo: cart read applies recalculated server discount.
- Expired/exhausted: applying code returns HTTP 422 reason.
- Client discount ignored: schemas strip discount fields and cart stores no discount amount.
