We need to use an app_factory for tinysearchengine if we want to use multiple uvicorn workers

[nitred]

If we would like to use multiple uvicorn workers for production then we need to have an app factory that uvicorn can use to spawn multiple instances of the app, one for each worker. Usually a new process is spawned for each worker and the workers & processes don't share any state.

However the app factory must take the config or config filename as an argument if the app is to be configured by the user.

Relates to #30

[nitred]

I don't think this is a priority at the moment but if ever the server needs to be beefier, using multiple workers is the easiest way to increase the ability of the server to handle more requests.