It's always nice to hear about someone writing a cool program just because they felt like doing so. Every time I think about writing a forum server, I find I return to the thought that Usenet was better, and that if I wanted a web forum, I'd consider putting a web front end on an NNTP server. That would among other things let people keep using NNTP clients (I like gnus.el) if they don't want to use a browser. There are a bunch of NNTP web clients already, but afaict they all needlessly suck.

You might take a look at Fossil (fossil-scm.org) which is not a forum, but is an all-in-one source control system, bug tracker, wiki, and web backend, all implemented as an SQLite client written in C. Its blurb says a scripting language might sound more attractive than C superficially, but in Fossil, SQLite does all the heavy lifting that would otherwise have been done with scripts. On my bottomless todo list I want to study its implementation sometime, since I think it would be a good example of "advanced" uses of SQL and SQLite. That is completely independent of Fossil's purpose as a VCS application. I'm ok at basic SQL but not so clueful about sophisticated uses.

One thing to watch out for is that since SQLite is an embedded DB, when you make an SQlite query, the overhead is basically a local subroutine call rather than the network traffic that a client/server DB would incur. That in a way changes the asymptotics of an application: particularly, the "N+1 queries" antipattern of some client/server DB apps is a perfectly ok design strategy for SQLite. I believe Fossil may rely on this in some way.

Every now and then I'm reminded of what could have been when someone submitted a patch to Dreamwidth (probably easily ported to Livejournal) that provided a bi-directional NNTP gateway that allowed posting. The Dreamwidth devs quite reasonably ended up regretfully rejecting it because there was no-one in the dev community who could maintain it, but sometimes when I'm reading through a community that generates 5000 messages a day I'm reminded that I could have had real kill files and proper tracking of new messages. Even close to 30 years later I haven't found anything as efficient for forums & such as a good newsreader was for netnews.

The D Language forum is a web forum/NNTP client/mail archive/irc bot/rss feed. You don't have to make an account to post. A friend of mine pointed out many years ago that it was basically the peak form of multi-protocol communications. If someone could implement convenient encryption it would be magic.