feat: multi-source artist images with thumbnails + grid integration

Complete rewrite of the artist image pipeline:

STORAGE:
- Migration 16: artist_images table tracking source, URL, path,
  primary flag, dimensions per image (up to 10 per artist)
- Directory structure: artist-images/{mbid[:2]}/{mbid}/ with
  primary.jpg + primary_sm.jpg/_md.jpg/_lg.jpg thumbnails
- Miss marker (.miss file) prevents re-fetching artists with no image

SOURCES (priority order):
1. MusicBrainz direct image relations (Wikimedia Commons)
2. Wikidata P18 property (Wikimedia Commons)
3. Wikipedia lead image (NEW — via Wikidata sitelinks → Wikipedia API)

Each source is checked, deduplicated, and the first available
image becomes the primary with sm/md/lg thumbnail generation
(100px/200px/400px, matching cover art tier sizes).

ASSET SERVING:
- /artist-images/ path registered with Wails asset handler
- Serves files via http.FileServer from the artist-images directory
- Same pattern as /covers/ for cover art

ARTIST MODEL:
- Artist struct gains ImageSmall/ImageMedium/ImageLarge fields
- resolveArtistImages does bulk MBID lookup → disk stat for each
- Populated in GetAllArtists and GetAllArtistsByLibrary

GRID VIEW:
- artists-view uses model URLs directly (no more base64 data URLs)
- Size selection based on imageSize * devicePixelRatio (like cover-grid)
- Removed batch GetArtistImages call and in-memory cache — no longer needed

frontend/wailsjs/go/explore/Service.d.ts

@@ -15,6 +15,8 @@ export function CoverArtURL(arg1:string):Promise<string>;
 
 export function GetArtistImageURL(arg1:string):Promise<string>;
 
+export function GetArtistImages(arg1:Array<string>):Promise<Record<string, string>>;
+
 export function GetArtistMBID(arg1:string):Promise<string>;
 
 export function GetThumbnail(arg1:string,arg2:string,arg3:string):Promise<string>;
@@ -44,5 +46,3 @@ export function StartIndexBuild():Promise<void>;
 export function StopIndexBuild():Promise<void>;
 
 export function TopRecordingsForArtist(arg1:string):Promise<Array<explore.LBTopRecording>>;
-
-export function GetArtistImages(arg1:string[]):Promise<Record<string, string>>;

frontend/wailsjs/go/explore/Service.js

@@ -26,6 +26,10 @@ export function GetArtistImageURL(arg1) {
   return window['go']['explore']['Service']['GetArtistImageURL'](arg1);
 }
 
+export function GetArtistImages(arg1) {
+  return window['go']['explore']['Service']['GetArtistImages'](arg1);
+}
+
 export function GetArtistMBID(arg1) {
   return window['go']['explore']['Service']['GetArtistMBID'](arg1);
 }
@@ -85,7 +89,3 @@ export function StopIndexBuild() {
 export function TopRecordingsForArtist(arg1) {
   return window['go']['explore']['Service']['TopRecordingsForArtist'](arg1);
 }
-
-export function GetArtistImages(arg1) {
-  return window['go']['explore']['Service']['GetArtistImages'](arg1);
-}