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/add_ssh_key.yml

@@ -0,0 +1,4 @@
+- name: Add user and authorized key
+  authorized_keys:
+    user: "{{ user }}"
+    key: "{{ key }}"