Unified voucher redemption: temporary owner authorization

[xiaoland]

Purpose

Support unified redemption: one redemption action can consume multiple Voucher entitlement units after explicit temporary authorization from every affected Voucher owner.

Relationship to #231

This is parallel to, not part of, #231.

#231 should keep the foundation compatible with this future mode, but the MVP should not implement multi-owner temporary authorization or batch redemption UX.

Voucher is one subtype of Entitlement. Unified redemption is a Voucher redemption mode, not another Voucher type.

Scope

• Temporary authorization grant from Voucher owner to redeemer.
• Grant expiry and revocation.
• Owner consent UI/API.
• Unified redemption action over multiple authorized Vouchers or entitlement units.
• Admin/merchant redemption audit recording both authorization evidence and consumption result.

Out of scope

• Changing the 电商MVP #231 Order / PaymentOrder / PackageOffer model.
• Moving payment splitting out of payable child Orders.
• Generic coupon marketplace behavior.
• Merchant POS integration.

Acceptance draft

• A Voucher owner can grant temporary redemption authorization to another user.
• Expired or revoked grants cannot be used.
• Unified redemption cannot consume another owner's Voucher without a valid grant.
• The redemption action is idempotent.
• Consumed item units still trace back to the original parent order allocations.

Related: #231, #238