Synchronous balance debit

The authoritative debit, called as the LAST step inside DeliveryVip’s order-creation transaction. A non-201 answer (or timeout) rolls the whole order back — the order is never born and the customer keeps shopping. Idempotency key is relatedOrderId (the order UUID): replaying the same relatedOrderId with the same amount must answer 201 with the SAME redemptionId; the same relatedOrderId with a DIFFERENT amount must be rejected with 422 code idempotency_conflict. amount is a JSON number (natural serialization, e.g. 25.0) and is always the value actually discounted on the order — it may be lower than what the customer originally reserved if the cart shrank. It also already honours the operator’s own capabilities.redemptionLimits: never below the declared minimum, never above the declared maximum. Percentage ceilings are computed over the order’s items (subtotal minus other cart discounts) — shipping and fees are never redeemable — so for operators whose own rule applies to the order total this amount can be lower than their ceiling, never higher. A 201 without redemptionId is treated as malformed and rolls the order back.