I've been trying this forum, and it's really cool project. Anyway, I wondered why you didn't go with SQLite or any other embbeded database. SQLite is very performant even for medium-sized websites, and with concurrent users, which SQLite deals by itself.

Why should it use any kind of Database if it is not neccessary?
A Database means additional administrative work, adds additionally dependencies and security Problems.
That makes Sense for Forums such as Discourse with thaousands of Themes and Millions of Threads and users.
But not a Forum with four or five themes a hundred of threads and some tens of users. and for that this Forum is perfekt.

I performed benchmarks with a Flatboard containing more than 100,000 messages without any problem.

I also chose the Json format because it is much more readable and reusable by any API.
  Flatboard doesn't use a member manager, but does it really need one?