Conversation block: support audio block as source for transcription

The conversation block only supports using a podcast player block, for adding player controls.

This works well for Anchor.fm users, and those who host their podcast elsewhere. But for users who host their podcast from their website, we should add support for other audio sources:

- core/audio block
- core/video block

Start by locating the conversation block and how it currently accepts a podcast player block as its source. Check how the core/audio and core/video blocks expose their media, then verify that both can provide audio for transcription while preserving the existing player-controls behavior.

Written by the indexing model from the issue text.