POST /api/playlists/system/for-you/refresh re-runs the system
playlist build synchronously for the calling user, then returns
the freshly-built For-You playlist's id, track count, and the
track IDs in playlist position order.
The frontend tile play button (next task) calls this endpoint
on click and enqueues the returned track_ids directly — one
roundtrip, no follow-up "list tracks" call needed for playback
to start. Same authenticated-user-only posture as the Discover
refresh from D-T3.
Returns playlist_id=null and track_ids=[] when the build
succeeded but the user's library yielded no eligible candidates
(degenerate empty-library). Returns 500 on actual build failure.
Tests cover 200 with shape (track_count matches len(track_ids))
and 401 without auth.
