Import history through the imported gifts API

For GiveCampus integrations and API.

Custom giving-history import uses the imported_gifts write API, not the read-only gifts export. Import/link constituents first and supply the GiveCampus person ID as gc_constituent_id; it is not a raw RE lookup ID. Core requirements include unique transaction_id, positive value, payment_date and explicit anonymous boolean. Map source/type/fiscal year and designation splits as applicable. Inspect row errors and duplicate IDs. If available, evaluate native NXT gift import before building a separate pipeline.