0.5.2: Run now for unscheduled playlists, Spotify /items migration, friendlier 403

- Run now (jobs.run_now): playlists without a daily sync time are never
  registered as scheduler jobs, so the old registered-job check 404'd. Fall
  back to running the playlist directly via pipeline_runner for any
  playlist:<name> key that isn't registered.
- Spotify removed GET /playlists/{id}/tracks in its February 2026 API changes
  in favor of /items (identical response shape). New apps are already 403'd on
  the old endpoint. Migrate both callers (app/services/spotify_client.py and
  pipeline/lib/spotify-retag.py) to /items. (The vendored sldl downloader uses
  its own bundled Spotify library and would need an upstream update when
  /tracks is fully removed.)
- Friendlier on-screen error for a Spotify 403 (check credentials + playlist
  must be public) and a README troubleshooting entry covering the 403, the
  public-playlist requirement, and the org-only extended-quota reality.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pipeline/lib/spotify-retag.py

@@ -46,7 +46,8 @@ def fetch_playlist(url: str, token: str) -> list[dict]:
         sys.exit(f"Could not parse playlist ID from {url!r}")
     pid = m.group(1)
     tracks = []
-    next_url = f"{API}/playlists/{pid}/tracks?limit=100"
+    # /items (not the removed /tracks) -- see February 2026 Spotify API changes.
+    next_url = f"{API}/playlists/{pid}/items?limit=100"
     while next_url:
         req = urllib.request.Request(
             next_url, headers={"Authorization": f"Bearer {token}"}