idx_timeline_event_persons_event_id and idx_timeline_event_documents_event_id
duplicated the leading column of their composite primary keys — Postgres already
serves timeline_event_id lookups from the PK index, so the extra indexes only
added write overhead. The inverse-side indexes (person_id, document_id) stay;
they cover the FK cascade path.
Deviates from the #774 task list ("all four FK columns") per PR #816 review.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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