- SpotifyClient: add _discogs lazy-load property, route _fallback to
DiscogsClient when configured (requires token, falls back to iTunes)
- web_server: _get_metadata_fallback_client returns DiscogsClient when
selected and token present
- Enhanced search: Discogs added as source tab with NDJSON streaming,
only available when token configured
- Alternate sources list includes Discogs when token is set
- Frontend: source labels, tab styling, fetch list all include Discogs
- Consistent with iTunes/Deezer pattern — same interfaces, same routing