Backup server: use backup_server_clients for keys and ssh config

Borg passphrase files are named by app; looping over all backup_clients let public-cloud-frontend overwrite the key of the live 'webserver' repo.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

templates/home/backup/.ssh/config.j2

@@ -1,7 +1,7 @@
-{% for client in backup_clients %}
+{% for client in backup_server_clients %}
 Host {{ client }}
     HostName {{ backup_host }}
     Port {{ backup_ssh_port }}
-    User {{ backup_clients[client].owner }}
+    User {{ backup_server_clients[client].owner }}
     IdentityFile {{ backup_home }}/.ssh/{{ client }}
 {% endfor %}