Auto-register borg repo on the controller per host

backup role now logs into borgcontroller and creates (or looks up) a
repository with alias=inventory_hostname, registering root's pubkey and
the requested storageSize. The resulting SSH URI is injected into the
borgmatic config so each host gets a remote-managed repo without manual
config.

backup_hosts entries gain a `storage_size_gb` field (stripped before
templating) and lose the manual `repositories` entry — the role fills it.
borgcontroller_{username,password} are expected from Semaphore secrets.

Also gitignores .claude/ local state.

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

roles/backup/templates/borgmatic.yaml.j2

@@ -1,3 +1,3 @@
 #jinja2: trim_blocks: True, lstrip_blocks: True
 # Managed by Ansible — do not edit by hand.
-{{ backup_hosts[inventory_hostname] | to_nice_yaml(indent=2, width=1000) }}
+{{ _borgmatic_config | to_nice_yaml(indent=2, width=1000) }}