God, I "love" Mastodon.
The server's cache filled up the entire HDD, and then from there put PostgreSQL in recovery mode
Honestly, Mastodon's caching sucks so hard, and it has no good way to do sanity checks like to automatically delete files, or make sure the HDD isn't filled up past a point.
I changed the crontabs to be more aggressive, but it appears the 'recommended' solution is to use S3 storage or something like that.
I'm legitimately debating what to do long term with the server ...