Could be shared by the various clients, allowing other apps to do things like add cards to Anki in a platform-independent way.

Some things to consider:

• Some form of access control, so that random apps or websites can't post changes to an open port. Global/more fine-grained?
• Protobuf, or JSON. JSON is less work for us at the Rust layer, easier for beginner developers to understand conceptually, and does not require extra build steps. Downsides are consumers could not rely on code generation for API, and the API would be described via Rust code instead of the somewhat-simpler .proto files.