Replace play_history with listening_events — one row per track exit,
kind (complete/play/skip) plus raw position/duration — and add
skip_count/last_skipped to audio_files beside play_count/last_played.
The classifier that writes these lands later (plan 021); this is the
schema it records into.
Also drop the dead queue.source_playlist_id column and remove the stale
references to the squashed migration chain in download_*.sql and
tagging_items.sql, declaring the missing download-request indexes inline.
Retiring a stale audio_files dropped every playlist entry to an empty
row: ON DELETE SET NULL ran before the phantom_* columns were filled,
whereas the manual rescan path populates them first. Run the same
phantom population inside the retire transaction, before the drop, only
when audio_files is among the tables going, so
ResolvePhantomTracksAfterScan can re-link the entries.
Closes#183