School identifier backfill includes Events linking

For GiveCampus constituent data and imports.

The school-level Backfill Constituent Identifiers action runs contribution identifier backfill and Events registration/attendee-to-person linking. Events linkage searches matching people by email and can fall back to external-identifier email mappings. It requires an existing person with a unique identifier. An email/ID mapping alone does not create that missing person; unmatched email does not guarantee a match. Import or correct the constituent/email data and rerun when appropriate. Shared email addresses can link multiple matching people, so review ambiguity. Do not describe this as a universal repair of every module.