Recurring gift identifiers after a CRM constituent merge

For GiveCampus constituent data and imports.

New installments initially copy the parent contribution’s external_identifier, and supported completion/matching paths can then run email-based identifier assignment. That service uses configured VMS/external-ID sources, preserves an existing value when no new match exists, combines multiple IDs, and propagates a changed recurring identifier across the chain. The public Subscription API routes are GET and serialize constituent_id from the parent contribution, not a writable subscription identifier endpoint. Correct authoritative mappings and have authorized staff run/review backfill with parent/installment samples.