fix(dashboard): use LEFT JOIN users in findContributorsPerDocument for deleted-user resilience

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

@@ -99,7 +99,7 @@ public interface AuditLogQueryRepository extends JpaRepository<AuditLog, UUID> {
                 COALESCE(u.color, '')                                            AS actorColor,
                 CONCAT_WS(' ', u.first_name, u.last_name)                       AS actorName
             FROM audit_log a
-            JOIN users u ON u.id = a.actor_id
+            LEFT JOIN users u ON u.id = a.actor_id
             WHERE a.kind IN ('ANNOTATION_CREATED', 'TEXT_SAVED', 'BLOCK_REVIEWED')
               AND a.document_id IN :documentIds
               AND a.actor_id IS NOT NULL