feat(discover): on-demand Lidarr artist art for suggestions

Out-of-library suggestion artists (artist_similarity_unmatched rows)
have no local art row, so the Discover card always showed a
placeholder. Resolve art on-demand from Lidarr's artist lookup,
matched by MBID (foreignArtistId == candidate_mbid), and pass the
remote image URL straight through — no caching (a suggestion may
never be viewed; the browser fetches the URL directly).

- suggestionView gains image_url (omitempty); handler resolves it via
  bounded-concurrency Lidarr LookupArtist, best-effort, never fails
  the request.
- Lidarr is the sole source: disabled / unreachable / no-match →
  empty → existing placeholder. (No inline TheAudioDB fallback — it's
  externally rate-limited and there's no cache to amortize it.)
- web: ArtistSuggestion.image_url?; SuggestionFeed passes it to
  DiscoverResultCard (already supports imageUrl).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/lib/api/types.ts

@@ -298,6 +298,7 @@ export type ArtistSuggestion = {
   name: string;
   score: number;
   attribution: SeedContribution[]; // up to 3 entries, ordered by contribution DESC
+  image_url?: string; // resolved on-demand from Lidarr; absent → card placeholder
 };
 
 // Mirrors internal/api/types.go HomePayload. All slices are non-null