GET /videos add fields selector parameter

Front ends that want metadata around events and presenters currently pay cost of transferring many video descriptions. It would be nice to have `?fields=presenter_id,event` for videos endpoint to reduce payload.

Locate the GET /videos entry point and its existing response tests, then read how video metadata is selected and serialized. Done means the endpoint accepts fields=presenter_id,event and returns only the requested metadata while retaining the current response behavior without the selector.

Written by the indexing model from the issue text.