use websockets instead of sse

[cstenglein]

Advantages:

• better integration with FastAPI https://fastapi.tiangolo.com/advanced/websockets/
• Authentication would be easier without workarounds
• no limitation on connection when not on HTTP/2 (see https://developer.mozilla.org/en-US/docs/Web/API/EventSource)
• Also possible to send messages directly (e.g. when forcing an update, getting other messages)