`_backup_db` names its copy `homelab.db.back-<APP_VERSION>`, so the 3.4.1 bump
renamed the backup `init_db` takes from `back-3.3.2` to `back-3.4.1`. The test
hardcoded its post-split copies as `back-3.3.1` / `back-3.3.2`, which happened
to overwrite that backup while the version was 3.3.2; at 3.4.1 it survives,
still carrying the columns this very boot is about to drop, and wins the
newest-first scan in `_pre_split_backup`.
The fixture was also unlike a real install, where the current version's backup
is taken after the columns are already gone — they went in the 3.3.0 boot, not
this one. Copy a post-split database over it too, under whatever the version is,
and the test stops breaking on every bump.
`_pre_split_backup` is unchanged: preferring the newest backup that still holds
the columns is right, the drop boot's own backup included.
ha-relevant: no
Pouzor
7aa322da35