Use FastAPI's native `StreamingResponse` for less dependencies. This
initially didn't work because I didn't include the additional header
specifying the content type, which is an event stream.
ref: N25B-110
The Python application exposes an endpoint /message for the UI to send
messages to.
It also exposes an SSE endpoint /sse for the UI to listen to. Every
second, the CB sends the current time to UI.
ref: N25B-107
ref: N25B-110