Backup coding system databases

[lucyb]

As far as I'm aware, we don't currently back up our coding system databases directly. If we lost these, they might be time consuming and tricky to restore. However, we do back up the Droplet, so we can restore the droplet and get a copy of the databases that way (see #2128).

We have one database per coding system release and generally consider them to be immutable. In total they take up 38G (at the time of writing) and will get larger over time.

We only have 155GB of disk space on dokku3 and are currently using half of it. So, it's probably not a good idea to backup the databases to the same machine. We should consider backing them up to Digital Ocean Spaces Object Storage and also storing the downloaded files there.

We should manage the backups in a similar way to #2151, by using a cron job. However, since the databases aren't written to we should be able to copy the files directly rather than use the sqlite cli.

Acceptance criteria

• Backup all coding system databases to Digital Ocean Spaces or other filesystem.
• Check that a local restore (NOT production) works as expected.
• Document the changes in an ADR.

[lucyb]

I'm closing this as I believe it will be made redundant by #2225 and a move over to Digital Ocean Block Storage.