feat(fediverse): player card for music posts (embed player, no raw mp3)
A post with audio now advertises a player card (twitter:player + og:video) pointing
at /embed?post=<slug>, so Mastodon shows an inline player that loads our embeddable
player — audio still streams via the gated /audio/stream, so no downloadable mp3 is
exposed. /embed?post=<slug> scopes the player to that post's tracks. Premium-gated
(the embed player is premium); non-premium falls back to the plain link card.
Co-Authored-By: Claude <noreply@…>
![(please configure the [header_logo] section in trac.ini)](/chrome/site/your_project_logo.png){kind=logo}
