Add backup role and manage_ssh_keys role

- Borgmatic backup role driven by per-host config in group_vars/all/backup.yml
- manage_ssh_keys role with add/remove paths; remove_ssh_key_playbook.yml uses it

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

roles/manage_ssh_keys/tasks/main.yml

@@ -0,0 +1,5 @@
+- include_tasks: add_ssh_key.yml
+  when: add_user | default(false)
+
+- include_tasks: remove_ssh_key.yml
+  when: remove_user | default(false)