Validates and redeems a voucher code within the authenticated user's campaign. On success, increments the redemption counter and publishes a voucher.VoucherRedeemed event for downstream discount processing.

The voucher itself carries no discount value — the campaign's discount type configuration determines what action the redemption triggers.

Some campaign types require a partner credential supplied via the X-User-Token header; when the campaign demands one and it is missing or rejected, the redemption fails with HTTP 401 before the voucher counter is decremented.