feat(fediverse): cover art on federated Audio attachments (AS2 icon)
A fedi_open track's Audio attachment now carries artwork as an AS2 icon (the
track cover, else the post cover), so receivers whose audio player supports
artwork (Mastodon's shows a thumbnail) render it instead of a blank tile. Purely
additive — a receiver that ignores icon is unaffected.
- src/services/ActivityPubService.js — addRow selects t.cover_url and attaches
icon {type:Image, mediaType, url} with post-cover fallback (track + playlist SELECTs).
- test/audio-attachment-art.test.js — track cover, post-cover fallback, no-cover no-icon.
- CHANGELOG(.nl/.de).md — "Cover art on openly shared audio" under Unreleased.
Co-Authored-By: Claude <noreply@…>