Donation note log

Add notes to individual donations — chargebacks, donor conversations, special circumstances, refund reasons, attribution to a specific solicitor, payment method nuances. Notes are timestamped, attributed to the user who wrote them, and searchable across all donations. Critical for the kinds of donations that need explanation: "this came in from a board member's check, we're crediting it to the gala campaign," "donor called to say it was meant for the scholarship fund."