Design recoverable backups

[junaga]

Define a backup and restore design for Debian hosts.

Goals:

• encrypted, versioned backups stored outside the host;
• protect retained backups from accidental deletion or a compromised client where practical;
• cover user data, system configuration, and recovery material;
• document restore steps and regularly test a restore.

This is intentionally deferred from Base setup until the storage location, trust boundary, retention policy, and recovery workflow are chosen.