Port Scala Backend to TypeScript

This replaces the Scala/gRPC backend with a Deno-based WebSocket API.

Obviously still needs a bunch of testing before it's good to go, but a TypeScript backend might be easier to maintain in the future.

I've set up an example deployment at https://buzzer.ente.ninja.

TODOs:

• fix reconnect issues
• update CI
• update REAMDE and deployment instructions
• make sure invalid requests don't crash the server (there's a big try-catch in the message handler, but sneaky type mismatches can probably produce broken transactions.)
• realistic beta testing
• update deployment step in CI