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>

group_vars/all/backup.yml

@@ -0,0 +1,15 @@
+---
+# Per-host borgmatic config. Keys must match inventory_hostname.
+# Hosts not listed here are skipped by the `backup` role.
+# The value under each host is rendered verbatim as the borgmatic
+# config file (see https://torsion.org/borgmatic/docs/reference/configuration/).
+backup_hosts: {}
+  # jim:
+  #   source_directories:
+  #     - /home
+  #     - /etc
+  #   repositories:
+  #     - path: ssh://user@backup.example.com/./backups/jim
+  #   keep_daily: 7
+  #   keep_weekly: 4
+  #   keep_monthly: 6