music: add per-guild queue with now/next/last, skip and queue

Tracks are queued in memory per guild, defaulting to append (last).
Playback auto-advances on TrackEvent::End/Error with a per-track token
so manual stops and skips cannot double-advance. /music queue shows the
current track followed by pending items. /stop clears the queue.

Cron audio now goes through the same MusicState instead of poking the
track-handle map directly.

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