Another followup: the broken upgrade process apparently also reset config.json somehow – so there was no URL for the site and the db password was gone. After fixing those settings the instance is now working again – but of course all of the other configurations that I’d set for the community have been blown away too, including people’s avatars, etc.
Note to self: just because the last several upgrades went seamlessly doesn’t mean you should skip taking a snapshot before running apt upgrade. 