Back up the daemon and restore its data

Back up the daemon and restore its data

The backup command asks the running daemon for an archive. A flushed
memory snapshot holds the write locks during the agent copy, and SQLite
VACUUM INTO saves the WhatsApp session without raw database files.
Compression runs after the write locks are released.

Restore validates the archive in a staging directory before it installs
the files. The token lock excludes a running daemon, and a nonempty
destination fails without replacement of its data.