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>

backup.yml

@@ -0,0 +1,8 @@
+---
+- name: Install borgmatic and deploy per-host config
+  hosts: all
+  become: true
+  tags: never,backup
+
+  roles:
+    - role: backup