Add ON DELETE behaviour to the two V1 FKs into persons (documents.sender_id
-> SET NULL, document_receivers.person_id -> CASCADE) and a real FK with
ON DELETE CASCADE on the transcription_block_mentioned_persons soft reference,
cleaning up pre-existing orphan mention rows first. The cascade stays strictly
at the join/reference layer and never reaches documents rows.
Proven by new Postgres-backed PersonRepositoryTest cascade tests (AC-1/2/3/8
plus the cascade-boundary document-survival guard). Rewrites the now-stale
V56 'no FK' comment.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
fd792f6d78