This was causing issues in which the last_child_created_at field for
stream particles was slightly after the actual particle's creation time,
and so comparing whether it's unseen merely via the playback markers
would be inaccurate.
This generates the transcript and shows the caption experience on the
client side for media particles. It also simplifies other side effects
that we must perform such as updating the `last_child_created_at` field
for stream and container particles.