Apply an authorised discount to a whole ticket

Applies a manager-authorised reduction to the WHOLE ticket and persists the traced record. This is the deliberate, one-off human decision — reach for it only when no automatic promotion or presented coupon covers the case, and never to reproduce an offer that should be a rule (POST /v1/promotions) or a code (POST /v1/coupons). You supply the id, which is the IDEMPOTENCY ANCHOR: a second post of the same id is rejected 409 rather than doubling the reduction, so generate one id per intended discount and retry with the same one. reason is a closed vocabulary (manual_markdown, promotional, loyalty_reward, damaged_goods, manager_goodwill) and note carries up to 280 characters of free-text justification. Express the reduction either as basis:'percent' with percentBps in BASIS POINTS (1000 = 10%) or basis:'amount' with amountCents in integer CENTS; the cart snapshot you pass is what the gross is computed from, and it is read, never mutated. The default policy caps a discount at 50% and at 50.00 € and refuses a reduction larger than the base — breaching any of those gives 422, which means ‘the request was well-formed but too deep’, not a malformed body. Applying a discount records it; it does not settle a sale, which stays POST /v1/checkout/finalize.