Two issues in the same path:
1. The shared discovery modal pre-renders one row per track from a
separately-fetched frontend track list, then the poll dropped any backend
result without a pre-rendered row (if (!row) return). When the frontend's
track fetch came back rate-limited/partial (~21) while discovery's own fetch
got all 59, the surplus results vanished. Now the modal CREATES a row for any
result lacking one, so authoritative backend results drive the list (fixes
all sources sharing the modal).
2. get_playlist hydrated a whole relationships page in one _get_tracks_batch
call, but Tidal caps filter[id] at 20/request, silently truncating larger
pages. Chunk to the cap like get_album_tracks already does.
Seam + regression tests (tests/test_tidal_playlist_batch_chunking.py).
