Single record endpoint

The backend should have an endpoint to retrieve information about a single endpoint based on its URI.

This is useful in itself, but the functionality it a good intermediate step for fetching the contents of a collection (#162). The collection graph only contains a reference to the record, not its contents. In order to generate a page with results, we need some way to get the contents of a record from a URI.

Start by locating the backend routing and the code that represents the collection graph, then trace how a record URI is currently handled. Define the single-record response from the existing record data, and verify that the endpoint can provide the contents needed when generating collection result pages.

Written by the indexing model from the issue text.