I have only found the source code for the Android and iOS application, but not for the server.

As an alternative to an Android TV, you could look into the Plasma Bigscreen project. Run it on whatever hardware you have available.

Plasma Bigscreen is an open-source user interface for TV’s. Running on top of a Linux distribution, Plasma Bigscreen turns your TV or setup-box into a fully hackable device. A big launcher giving you easy access to any installed apps and skills. Controllable via voice or TV remote.

I’m currently looking into Plasma Bigscreen as a desktop environment for an open source smart TV, it looks promising.