fix(db): V51 backfills annotation_id on block comments and notifications

Previously issued block-comment notifications were stored with
annotation_id=NULL because CommentService.postBlockComment did not
populate DocumentComment.annotationId. Now that the code fix is in
place, existing rows need to be filled in so legacy notifications
can also carry the query param that the frontend deep-link flow
expects.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/main/resources/db/migration/V51__backfill_block_comment_annotation_id.sql

@@ -0,0 +1,24 @@
+-- Backfill annotation_id on block comments and their notifications.
+--
+-- Before the upstream fix, CommentService.postBlockComment did not set
+-- DocumentComment.annotationId, so block comments were stored with
+-- annotation_id = NULL and every notification built from them inherited
+-- that NULL (see NotificationService.notifyMentions/notifyReply).
+--
+-- The frontend deep-link flow needs annotationId in the URL query string
+-- to open the correct annotation panel and scroll to the comment.
+-- Without this backfill, previously issued notifications would still
+-- carry annotation_id = NULL even after the code fix lands.
+
+UPDATE document_comments dc
+SET annotation_id = tb.annotation_id
+FROM transcription_blocks tb
+WHERE dc.block_id = tb.id
+  AND dc.annotation_id IS NULL;
+
+UPDATE notifications n
+SET annotation_id = dc.annotation_id
+FROM document_comments dc
+WHERE n.reference_id = dc.id
+  AND n.annotation_id IS NULL
+  AND dc.annotation_id IS NOT NULL;