Delete the dead grouped-download block, the planner, and the music service hooks; Song now flows through the one download loop, gets tagged via tag_file, and answers -w with {disc}x{track} keys.
BREAKING CHANGE: GROUP_AUDIO_DOWNLOADS, get_music_track_options and get_music_collection_label are gone. A music service returns one Audio track per quality and lets the framework choose.
Adds GET /api/download/jobs/{job_id}/events with snapshot, status, progress and terminal events, query-param auth for EventSource in both serve modes, atomic progress-file writes, and stream-safe CORS and compression middleware.
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