Backup server: incremental mirror, client subset, configurable compression

- local_backup rsynced every repository into an empty folder each night (full transfer); keep a persistent mirror (backup_server_mirror) and rsync --delete into it
- backup_server_clients: clients handled by the server (app names must be unique, e.g. two 'webserver' apps collided)
- backup_server_local_skip: apps that are pruned remotely but not mirrored
- backup_server_compression: borg repos are encrypted already, allow 'none'

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

tasks/server.yml

@@ -75,7 +75,7 @@
     group: "{{ backup_group }}"
     mode: "0750"
   loop:
-    - "{{ backup_remotes_folder }}"
+    - "{{ backup_server_mirror | default(backup_remotes_folder) }}"
     - "{{ backup_location }}/{{ backup_app }}"
 
 - name: setup borg local repository