New GET /api/downloads/task/<id>/detail merges the live download task with its
library_history row (the data the Download History cards show) into one payload
the upcoming track-detail modal renders: status kind, title/artist/album,
source, quality, final location, AcoustID verdict, and expected-vs-downloaded.
Assembly + status classification live in core/downloads/track_detail.py as a
pure, importable build_track_detail()/classify_status_kind() (9 unit tests);
the endpoint is thin glue that looks up the matching history row by track.
e4bbcfda1b