fix(audit): backfill COMMENT_ADDED and MENTION_CREATED events

Comments created before audit logging was added in 428c63a2 have no
corresponding audit_log rows, so the Chronik activity feed (which
reads exclusively from audit_log) cannot surface them in "Alle" or
"Für dich", even though the fix from #295 is wired up correctly.
V50 inserts the missing events idempotently from document_comments
and comment_mentions.

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

backend/src/main/resources/db/migration/V50__backfill_comment_audit_events.sql

@@ -0,0 +1,39 @@
+-- Backfill COMMENT_ADDED and MENTION_CREATED audit events for comments
+-- created before audit logging was added in commit 428c63a2.
+-- Without these rows the Chronik activity feed (which reads exclusively from
+-- audit_log) cannot surface pre-existing comments in "Für dich" or "Alle".
+
+INSERT INTO audit_log (id, happened_at, actor_id, kind, document_id, payload)
+SELECT
+    gen_random_uuid(),
+    c.created_at,
+    c.author_id,
+    'COMMENT_ADDED',
+    c.document_id,
+    jsonb_build_object('commentId', c.id::text)
+FROM document_comments c
+WHERE NOT EXISTS (
+    SELECT 1 FROM audit_log a
+    WHERE a.kind = 'COMMENT_ADDED'
+      AND a.payload->>'commentId' = c.id::text
+);
+
+INSERT INTO audit_log (id, happened_at, actor_id, kind, document_id, payload)
+SELECT
+    gen_random_uuid(),
+    c.created_at,
+    c.author_id,
+    'MENTION_CREATED',
+    c.document_id,
+    jsonb_build_object(
+        'commentId', c.id::text,
+        'mentionedUserId', m.user_id::text
+    )
+FROM comment_mentions m
+JOIN document_comments c ON c.id = m.comment_id
+WHERE NOT EXISTS (
+    SELECT 1 FROM audit_log a
+    WHERE a.kind = 'MENTION_CREATED'
+      AND a.payload->>'commentId' = c.id::text
+      AND a.payload->>'mentionedUserId' = m.user_id::text
+);