Map donation incentives to CRM benefits explicitly

For GiveCampus integrations and API.

An incentive is a selectable donor benefit with a configured fair-market value; it is not a CRM inventory/quid code mapping by itself. The Gifts API incentive object supplies its id, headline, description, market_value and tax_deductible_amount. Map the stable incentive ID to the institution’s CRM benefit/inventory record in the integration and handle fulfillment there; do not assume automatic stock/quid creation. The API calculation is gift value minus configured incentive FMV, while the standard receipt floors its displayed deductible amount at zero, so integrations must not assume the two renderings are identical in an overvalued-benefit edge case. The institution determines appropriate benefit values and receipt treatment.