GrapheneOS' App Store client is available on Github, but I couldn't find the code of the backend, which stores the APKs and provide the updates. Is there any reason why this code isn't open? Some of the backends, typically for Auditor, are already opened as well.
I believe it might be handled by the same server as the release one (https://github.com/GrapheneOS/releases.grapheneos.org), but I don't see any support structure foe metadata for instance.