It’s called Ditto. It’s a Nostr-first backend, but it will introduce an API to obtain a NIP-05 address on the domain it’s hosted on, and if you do it will enable ActivityPub support on your account. And of course it supports Mastodon API.

My prototype is a pure client to a relay pool (no database), but I plan to support Gossip model and I’m trying to figure out how to make user feeds consistent. I know I’ll need to store some data. I want to build it in a way that the database is a cache that can expire and be rebuilt from relays. I’m tempted to use Redis because it will be a lot easier than LMDB, but I’m torn because it would increase installation complexity. Depending on how much data we store it might need to become a relay itself, then I can do advanced queries. I’m still mulling on the various technical/product trade offs, but the core idea is Ditto.