📄️ Check a voucher code
Checks that the code is live: it exists, is unused, its promotion is active and inside its dates, and quota is left. It answers `200` with `valid: false` and a `reason` for any code that is not live, and never `404`. When `valid` is true, `restrictions` shows the rules of the promotion. Outlet, customer and minimum purchase rules are not checked here; they are checked when the order is created, so a live code can still be refused for an order. Codes are never listed. Needs the order discount voucher feature in the merchant's plan, otherwise `403 PLAN_FEATURE_REQUIRED`.