Fix /champions

[Bekabyx]

Champions is not currently working as expected - it's returning all finalists.

I'd like to overhaul this to be based off of Year in its initial query and have a format like

{
    "year": 2022,
    "bracket_date": "2022-10-11",
    "contestants": {
        /* basic bear serializer here */
    }
}

Obviously, I'd also like to address the broken logic.

I'd also like to overhaul the (currently working) Finalists endpoint to use this same format. In this case, the contestants field should be a list rather than a single object.

May need some more thought to maximise code reuse and cleanliness, but jotting my ideas down here.