Reduce the friction for new dev to make a contribution

[neolectron]

In order to reduce the initial setup to work on the project, I think REDIS_URL env should be optional in development (not prod).

If there's no redis URL the keyv library do use in-memory storage by default.

It would also allow contributors to quickly setup the project even without having docker on their machine.
It might drive more contributors as it would be really fast to have your project up and running.