Automate DB backups

[Minoru]

SQLite has an API for this: https://sqlite.org/backup.html

Considering we're using SQLite in WAL mode, it probably makes sense to do the backup in one step. This only blocks the checkpointing process, which shouldn't run all that often anyway.

[Minoru]

VACUUM INTO might be an even better fit here: https://sqlite.org/lang_vacuum.html#vacuuminto