Resolve an arbitrary article URL to its Storyflo audio. Used by the newsletter embed badge — newsletter authors drop a one-line <a> or <iframe> linking to /embed/badge?u=<url> and Storyflo handles the rest:

• if we already rendered audio for that URL, return it immediately
• if not, queue a render-on-demand (priority lane) and return status='rendering' so the badge can poll until ready
• tag every produced article with origin='subscribed' and sender_email=<host> so per-publisher rev-share accrues from the first listen, no claim flow required up front