Add store abstraction

[Ozoniuss]

Having the bussiness logic directly inside the handlers (including the database interaction) is not something I'm proud of. This is the most basic "transaction script" approach I started with because I wasn't really writing tests back then, and now testing is a pain in the ass. If I'm going to stick with the layered architecture I'd like to at least have an application layer on top of this.

Having the store abstraction would be reusable if I'm moving towards ports & adapters at some point.