Turns out Jellyfin uses a really esoteric form of Musicbrainz ID for tracks. Instead of using the recording ID, it uses the specific hash for a given version of a recording. A noble effrot to be specific, but we'd much rather use Mopidy's recording ID when it's available. Thus, we'll use Jellyfin's ID if that's all we have, but if we scrobble the same track from Mopidy, overwrite the value.