> Say there's a thousand threads and a 500 users, that's only 61kB
You do realize that this is an SQL database, not a C array? You don't just need a boolean... you need a <thread id, user name> pair, which is 24 bytes at the moment. Also realize that this sort of setup is not sustainable in the long run without pruning the data (the data set just keeps growing and growing).
> Would it be possible to add a single button that refreshes the logout time?
Good point. I can really imagine a "Mark all threads as 'read'" button, however it would also apply to every other section as we only use one single variable to compare times. I'm not sure how to name the button nor where to place it, though.