Admin-only handler. Calls tracks.RemoveTrack with the unmonitor flag
parsed from the query string. ErrNotFound -> 404; everything else
unexpected -> 500. Lidarr-side failures during unmonitor flow as
lidarr_unmonitor_failed: true in the success envelope (non-fatal —
the file + DB delete already completed).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
