Automerge is an excellent library, with a great API, not just in Rust, but also Javascript and C.

> All you need for the backend is key-value storage with range/prefix queries;

This is true, I was able to quickly put together a Redis automerge library that supports the full API, including pub/sub of changes to subscribers for a full persistent sync server [0]. I was surprised how quickly it came together. Using some LLM assistance (I'm not a frontend specialist) I was able to quickly put together a usable web demo of synchronized documents across multiple browsers using the Webdis [1] websocket support over pub/sub channels.

[0] https://github.com/michelp/redis-automerge

[1] https://webd.is/