Wire backup_borg conditional restart variable into group_vars/matrix_servers

Replace hardcoded restart_necessary: true with the computed
backup_borg_restart_necessary variable that the role already exposes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

group_vars/matrix_servers

@@ -278,7 +278,7 @@ devture_systemd_service_manager_services_list_auto: |
     ([{
       'name': (backup_borg_identifier + '.timer'),
       'priority': 5000,
-      'restart_necessary': true,
+      'restart_necessary': (backup_borg_restart_necessary | bool),
       'groups': ['matrix', 'backup', 'borg'],
     }] if backup_borg_enabled else [])
     +