Backend (work in progress, don't merge)

Future plan for the OperationFinder might be to make it chainable:

```ruby
class Queries::Post::FindByAuthor
  # ...
end

backend.queries.post.find_by_author(...)
```

If we then make it so that backend also works like this, we could do `backend.post.queries.find_by_author` or whatever style the user desires. Doesn't even need to be split into queries/commands/interactors at that point, you could introduce more or less categories as desired.