refactor(audit): document MIN invariant in aggregated CTE comment

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/audit/AuditLogQueryRepository.java

@@ -71,6 +71,7 @@ public interface AuditLogQueryRepository extends JpaRepository<AuditLog, UUID> {
                     COUNT(*)::int                                                 AS count,
                     BOOL_OR(s.kind = 'MENTION_CREATED'
                         AND s.payload->>'mentionedUserId' = :currentUserId)       AS you_mentioned,
+                    -- COMMENT_ADDED/MENTION_CREATED always have is_new_session=1, so each group has one row and MIN collapses to that row payload
                     MIN(s.payload::text)::jsonb                                   AS payload
                 FROM sessions s
                 GROUP BY s.kind, s.actor_id, s.document_id, s.session_id