0.6.3: Parse Spotify's renamed /items response shape (new apps get item, not track)

Spotify's February 2026 changes did not just move /playlists/{id}/tracks to
/items: for apps on the new behavior the per-entry payload key was renamed
from "track" to "item" (tracks.tracks.track -> items.items.item). Extended
Quota Mode (grandfathered) apps keep the old key, which is why this never
reproduced locally. Every parser in alembic read only "track", so on a new
app each entry looked like a null/local track and was silently skipped: the
CSV came out header-only, sldl no-opped with exit 0, and the playlist page
showed an empty tracklist. Confirmed live on a new app against a playlist
the connected account owns.

All /items consumers (spotify_client.py, spotify-playlist-csv.py,
spotify-retag.py) now parse both key names, and the fields query filter is
gone: it selects by key name, so filtering on track(...) is itself what
returned empty pages on the renamed shape.

Also per the migration guide, new apps only receive playlist contents for
playlists the connected account owns or collaborates on; other playlists
return metadata with no items field at all (public is no longer enough).
That case now raises a pointed error (UI and CSV fetch) instead of reading
as an empty playlist, run-playlist.sh logs the fetched track count and warns
loudly when it is zero, and the README guidance is updated to match.

New tests pin get_playlist_tracks against both response shapes, the
metadata-only error, and pagination.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

pipeline/bin/run-playlist.sh

@@ -84,6 +84,18 @@ if ! SPOTIFY_CLIENT_ID="$SPOTIFY_CLIENT_ID" SPOTIFY_CLIENT_SECRET="$SPOTIFY_CLIE
   exit 1
 fi
 
+# Surface the fetched count in the timestamped summary. Zero tracks from a
+# successful fetch is almost never a truly empty playlist -- it usually means
+# Spotify hid the contents from the connected account (see the pointed errors
+# in spotify-playlist-csv.py), so call it out instead of letting sldl no-op
+# with a clean exit 0.
+TRACK_COUNT=$(($(wc -l < "$CSV_FILE") - 1))
+if [[ "$TRACK_COUNT" -le 0 ]]; then
+  log "WARNING: Spotify returned 0 visible tracks for this playlist -- nothing to download. If the playlist isn't actually empty, the connected Spotify account can't see its contents (it must own or collaborate on the playlist for newly created Spotify apps)."
+else
+  log "Fetched $TRACK_COUNT track(s) from Spotify"
+fi
+
 # ==== Run sldl (vendored binary, subprocess of this container) ====
 log "Running sldl for: $PLAYLIST_NAME"