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>

internal/api/suggestions.go

@@ -1,12 +1,15 @@
 package api
 
 import (
+	"context"
 	"net/http"
 	"strconv"
+	"sync"
 
 	"github.com/jackc/pgx/v5/pgtype"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/apierror"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/lidarr"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/recommendation"
 )
 
@@ -16,6 +19,11 @@ type suggestionView struct {
 	Name        string                 `json:"name"`
 	Score       float64                `json:"score"`
 	Attribution []seedContributionView `json:"attribution"`
+	// ImageURL is resolved on-demand from Lidarr (out-of-library
+	// artists have no local art row). Omitted when Lidarr is disabled
+	// or has no match — the client falls back to a placeholder. Not
+	// cached: a remote URL Lidarr surfaced, fetched by the browser.
+	ImageURL string `json:"image_url,omitempty"`
 }
 
 type seedContributionView struct {
@@ -80,5 +88,52 @@ func (h *handlers) handleListSuggestions(w http.ResponseWriter, r *http.Request)
 			MBID: s.MBID, Name: s.Name, Score: s.Score, Attribution: attr,
 		})
 	}
+	h.resolveSuggestionArt(r.Context(), out)
 	writeJSON(w, http.StatusOK, out)
 }
+
+// resolveSuggestionArt fills ImageURL on-demand from Lidarr's artist
+// lookup, matched by MBID (foreignArtistId). Best-effort and cache-free:
+// Lidarr is the only source — when it's disabled, unreachable, or has
+// no match for a candidate, that entry keeps an empty ImageURL and the
+// client renders its placeholder. Lookups run with bounded concurrency
+// so a full Discover page doesn't serialize ~12 round-trips. Never
+// fails the request; the suggestions list is the contract, art is a
+// nicety.
+func (h *handlers) resolveSuggestionArt(ctx context.Context, views []suggestionView) {
+	if len(views) == 0 {
+		return
+	}
+	cfg, err := h.lidarrCfg.Get(ctx)
+	if err != nil || !cfg.Enabled || cfg.BaseURL == "" || cfg.APIKey == "" {
+		return // Lidarr off / unconfigured → placeholders only
+	}
+	client := lidarr.NewClient(cfg.BaseURL, cfg.APIKey)
+
+	const maxConcurrent = 6
+	sem := make(chan struct{}, maxConcurrent)
+	var wg sync.WaitGroup
+	for i := range views {
+		if views[i].MBID == "" || views[i].Name == "" {
+			continue
+		}
+		wg.Add(1)
+		sem <- struct{}{}
+		go func(idx int) {
+			defer wg.Done()
+			defer func() { <-sem }()
+			results, lerr := client.LookupArtist(ctx, views[idx].Name)
+			if lerr != nil {
+				return // best-effort: no art on lookup failure
+			}
+			for _, res := range results {
+				if res.MBID == views[idx].MBID && res.ImageURL != "" {
+					// Distinct slice index per goroutine → race-free.
+					views[idx].ImageURL = res.ImageURL
+					return
+				}
+			}
+		}(i)
+	}
+	wg.Wait()
+}