Update event guest identifiers after an identifier backfill

For GiveCampus constituent data and imports.

The current school-level external-identifier backfill does run an Events registration/attendee-to-person linking step after contribution backfill. It is not a guarantee every guest matches: inspect imported IDs/emails, event person links and ambiguous/missing matches. The date-limited contribution backfill and event linking are separate operations. Support must review the specific guest and run to explain the still-empty field.