feat(fediverse): federate on save+scheduler, delivery retry-queue, music as listen-link
#1 Posts now federate to followers not only on create, but also when a draft/
scheduled post becomes published (editor save) and when the Scheduler flips a
scheduled post live — previously those silently didn't reach followers.
#2 Delivery retry-queue (ap_delivery): a failed delivery (down server/timeout) is
queued and retried with backoff (1/5/15/60/180/360 min, 6 tries) by a worker,
instead of fire-and-forget. Signing key re-derived from the slug, never stored.
#3 Music posts: audio shortcodes federate as a '🎵 listen on the site' link to the
post (protected player) instead of the raw mp3 — keeps Klonkt's audio friction
intact (no downloadable file handed to Mastodon).
Co-Authored-By: Claude <noreply@…>
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