Child table for @-mentions inside transcription block text. Each row binds
one block to one person via personId + displayName; the literal "@DisplayName"
stays in block.text. No FK on person_id so deleted persons degrade gracefully
to plain unlinked text rather than cascade-deleting the block. Indexed on
person_id for the future "blocks mentioning person X" query and on block_id
for the @ElementCollection load.
Schema choice diverges from document_comments.comment_mentions (many-to-many
to AppUser): the latter cascades, this one degrades. Mirrors the established
UserGroup.permissions / group_permissions @ElementCollection pattern.
Refs #362
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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