I wasn't able to build Briar Headless from source for Debian. It seems that Kotlin isn't yet supported in Debian, but Briar is relying heavily on it. Therefore, I decided to create a simple binary package that I'll upload to my own apt repository until Kotlin support landed in Debian.
First steps were analogous to https://salsa.debian.org/xmpp-team/sms4you/-/commit/6dcf2ccc13d4d2f962fafb96ba9a9ac54ce9fa18.
Related to #1739 (moved), but doesn't close it.