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.
yonlu
55473dd52a