Purged SQLite from the code and implemented an in-memory store instead

I’ve made an interface called CardStore so I can implement multiple ways of storing cards. In future, I might use this to benchmark and compare different methods but for now I’m using an in-memory store so I can quickly test things out.

I’ve made a plan for the persistent card storage, but I’ll get to that later.

For now I’m going to work on the initial sync with Anki. This will be much easier than the partial sync (where cards have been added, edited, removed, and reviewed).