# Architect — F-020 Shipping module

## Feature
F-020 adds server-side shipping quotation behind `ShippingService.calculate(cart, address)`.

## Design

### Module boundaries
Create `project/src/modules/shipping/` with domain/application/infrastructure/api/tests. Shipping owns zones and methods; it does not own cart data. Callers pass a cart summary and destination address into the public service.

### Data model
Add migration `015_shipping.js`:
- `shipping_zones`: id, name, country, postal_code_prefix nullable, active.
- `shipping_methods`: id, zone_id, name, base_cost_cents, free_shipping_threshold_cents nullable, active.

### Rules
- Address matching uses country plus optional postal-code prefix. Prefer the most specific prefix.
- `ShippingService.calculate(cart, address)` returns the cheapest active method in the matching zone.
- If cart total is at or above method threshold, shipping cost is zero.
- If no zone/method matches, throw a domain error mapped to HTTP 422.
- Carrier adapter is an interface only; no real carrier API.

### API
- Admin create/update can be minimal for test setup: `POST /shipping/zones`, `POST /shipping/methods`.
- Public/authenticated calculation endpoint: `POST /shipping/calculate` with cart total/address payload.

## Acceptance trace
- Known zone returns shipping cost.
- Unknown zone returns HTTP 422 with clear code.
- Cart above free shipping threshold returns zero cost.
