Create backend-go which can swap the main tutorial backend written in Django

To extend the tutorial for the Go ecosystem. Should be a drop-in replacement for the current Django backend (to make it clear – Django should still be kept). And it would be nice to see how a faster language for the same task may improve the latencies further.

Start by reading the existing Django backend and the tutorial documentation to identify the interfaces and chat flow that a replacement must preserve. Done means a Go backend can be swapped in while Django remains available, with latency results that allow comparison between the two implementations.

Written by the indexing model from the issue text.