[release] v1 release for backend

[LeoHChen]

For the v1 API, since it is mainly used by the published project and we have no intention to keep working on it. So I think we shall create a release branch of v1 to freeze the code so that later change won't worry about the breakage.

However, I understand there is a maintenance cost to run the v1 backend server. Let's discuss with @leeren on how to keep a small v1 backend running for longer time.

[edisonz0718]

@leeren I think even the codebase for v1 can be separate. We could make another repo for it and find a lighter-weight hosting solution if you feel it makes it easier to maintain. The code and logic are pretty much standalone.