Calculate donor fee-coverage percentage

For GiveCampus reporting.

Export the intended successful gift population with gift ID, donor identifier/email and donor_covered_payment_fee. For a donor-level rate, define a consistent unique-donor key and count donors with at least one positive coverage amount divided by all unique donors in that same eligible scope, times 100. A gift-level rate instead counts gifts and must be labeled separately. Deduplicate split-report rows by gift ID and specify handling of refunds/installments; positive recorded coverage measures payment coverage, not every person who was offered or clicked the option.