Add BorgBase offsite backup repository (#142)

## Summary
- Adds BorgBase as a second borgmatic repository for offsite backups (US region, append-only)
- SSH key managed via 1Password, deployed to indri by Ansible
- Borgmatic `ssh_command` configured to use the dedicated BorgBase key
- BorgBase host key pinned in known_hosts via Ansible

## Post-merge deployment steps
1. Provision borgmatic: `mise run provision-indri -- --tags borgmatic`
2. Initialize the BorgBase repo: `ssh indri 'mise x -- borgmatic init --encryption repokey --repository borgbase-offsite'`
3. Export and store the borg repokey: `ssh indri 'borg key export ssh://k04ljcd7@k04ljcd7.repo.borgbase.com/./repo'` → save to 1Password
4. Verify first backup: `ssh indri 'mise x -- borgmatic create --repository borgbase-offsite --verbosity 1'`

## BorgBase setup (already done)
- Account created, API token in 1Password (`borgbase` item in blumeops vault)
- SSH keypair generated, stored in 1Password, public key uploaded to BorgBase (ID: 200815)
- Repository `indri-borgmatic` created (ID: k04ljcd7, US region, append-only, 2-day alert)

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/142

docs/changelog.d/feature-borgbase-offsite-backup.feature.md

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+Add BorgBase offsite backup repository for 3-2-1 backup strategy