feat: artist images from MusicBrainz/Wikidata/Wikimedia Commons

Add ArtistImageProvider that resolves artist MBIDs to photo URLs:
1. MB url-rels 'image' type → extract Commons filename → thumb URL
2. MB url-rels 'wikidata' type → Wikidata P18 property → thumb URL
3. No image → falls back to initial-letter avatar

Wikimedia Commons thumb URLs constructed via MD5 hash bucketing
(standard Commons URL scheme). Results cached in explore_cache
with 30-day TTL — subsequent lookups are instant.

Frontend: search results and artist detail page show artist photos
in the circular avatar when available. Images load async and
replace the initial-letter fallback on arrival. Artist detail page
fires the image fetch alongside the other 4 parallel data loads.

Architecture supports adding more sources (fanart.tv, etc.) by
extending the resolve() method's source chain.

frontend/src/components/explore-artist-details/explore-artist-details.ts

@@ -6,6 +6,7 @@ import {
     BrowseReleaseGroups,
     TopRecordingsForArtist,
     SimilarArtists,
+    GetArtistImageURL,
 } from '@go/explore/Service';
 import type {
     MBArtist,
@@ -88,6 +89,7 @@ export class ExploreArtistDetails extends LitElement {
     @state() private errorReleases = '';
     @state() private similarArtists: LBSimilarArtist[] = [];
     @state() private loadingSimilar = true;
+    @state() private artistImageURL = '';
 
     /* ── Styles ── */
 
@@ -149,6 +151,13 @@ export class ExploreArtistDetails extends LitElement {
                 user-select: none;
                 flex-shrink: 0;
                 line-height: 1;
+                overflow: hidden;
+            }
+
+            .artist-avatar img {
+                width: 100%;
+                height: 100%;
+                object-fit: cover;
             }
 
             .artist-info {
@@ -484,7 +493,7 @@ export class ExploreArtistDetails extends LitElement {
             `[explore-artist] loading: "${this.artistName}" (${mbid})`,
         );
 
-        // Fire all four requests in parallel — each section is independent.
+        // Fire all five requests in parallel — each section is independent.
         const [artistResult, tracksResult, releasesResult, similarResult] =
             await Promise.allSettled([
                 this.fetchArtist(mbid),
@@ -493,6 +502,9 @@ export class ExploreArtistDetails extends LitElement {
                 this.fetchSimilarArtists(mbid),
             ]);
 
+        // Artist image is fire-and-forget — doesn't block the page.
+        this.fetchArtistImage(mbid);
+
         const summary = [
             `artist=${artistResult.status}`,
             `tracks=${tracksResult.status}`,
@@ -562,6 +574,17 @@ export class ExploreArtistDetails extends LitElement {
         }
     }
 
+    private async fetchArtistImage(mbid: string) {
+        try {
+            const url = await GetArtistImageURL(mbid);
+            if (url) {
+                this.artistImageURL = url;
+            }
+        } catch {
+            // No image available — avatar stays as initial letter.
+        }
+    }
+
     /* ── Navigation ── */
 
     private navigateBack() {
@@ -709,7 +732,12 @@ export class ExploreArtistDetails extends LitElement {
                     class="artist-avatar"
                     style="background: hsl(${hue}, 45%, 35%)"
                 >
-                    ${this.getInitial(this.displayName)}
+                    ${this.artistImageURL
+                        ? html`<img
+                              src="${this.artistImageURL}"
+                              alt="${this.displayName}"
+                          />`
+                        : this.getInitial(this.displayName)}
                 </div>
                 <div class="artist-info">
                     <h1 class="artist-title" title="${this.displayName}">