feat(transcription): V56 migration adds transcription_block_mentioned_persons sidecar

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>

backend/src/main/resources/db/migration/V56__add_transcription_block_mentioned_persons.sql

@@ -0,0 +1,25 @@
+-- Sidecar table for @-mentions inside transcription_blocks.text.
+-- Each row is one (block_id, person_id, display_name) tuple emitted by the
+-- typeahead in the transcription editor. block.text contains the literal
+-- "@DisplayName" — the UUID lives only here so historical text stays clean.
+--
+-- Schema choice: child table via @ElementCollection (mirrors the established
+-- UserGroup.permissions / group_permissions pattern), NOT JSONB. The "show
+-- all blocks mentioning person X" query on the person detail page joins on
+-- the indexed person_id column — equally fast as JSONB GIN containment, no
+-- new dependency. document_comments.comment_mentions stays as a many-to-many
+-- to AppUser; the divergence is intentional: Person mentions need lazy
+-- degradation when a person is deleted (no FK), while user mentions don't.
+--
+-- No FK on person_id: when a Person is deleted we want @Auguste Raddatz to
+-- remain visible as plain unlinked text inside the transcription rather than
+-- vanishing or cascade-deleting the block.
+
+CREATE TABLE transcription_block_mentioned_persons (
+    block_id      UUID          NOT NULL REFERENCES transcription_blocks(id) ON DELETE CASCADE,
+    person_id     UUID          NOT NULL,
+    display_name  VARCHAR(200)  NOT NULL
+);
+
+CREATE INDEX idx_tbmp_person_id ON transcription_block_mentioned_persons(person_id);
+CREATE INDEX idx_tbmp_block_id  ON transcription_block_mentioned_persons(block_id);