Data export and restore

A backup is a versioned, checksummed file of the user's tables that this app (and only a backup) can restore; a restore checks the whole file first, keeps a safety copy, then replaces every table in one transaction or changes nothing; a CSV export is guarded against broken rows and formula injection; files leave or enter the app only through ports, from a press; tests prove the round trip byte for byte.

Rules that must hold