Extends the watchlist export to the full library. The exporter is now general
(core/exports/artist_export.py, renamed from watchlist_export) — adds tidal/qobuz
links and an extra_fields passthrough, so the library export also carries
lastfm/genius URLs + soul_id, and an optional "library counts" toggle adds owned
album/track counts per artist.
- GET /api/library/artists/export?format=&links=&contents= — pulls every artists
row, normalizes onto the canonical *_artist_id keys, optionally GROUP-BY counts
for album/track totals.
- The export modal is now openArtistExportModal(scope): "Export Library" button in
the library header + the existing "Export" on the watchlist bar (a thin wrapper).
Library mode shows the extra "library counts" toggle.
Tests (11): builder across formats + the new tidal/qobuz links + extra_fields
columns; watchlist + library endpoint wiring. 64 integrity green; ruff clean.
