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

backend/app.go

@@ -8,6 +8,8 @@ import (
 	"errors"
 	"fmt"
 	"log/slog"
+	"net/http"
+	"path/filepath"
 
 	wailsruntime "github.com/wailsapp/wails/v2/pkg/runtime"
 
@@ -23,6 +25,7 @@ import (
 	"yellowjacket/backend/playlist"
 	"yellowjacket/backend/profiling"
 	"yellowjacket/backend/queue"
+	"yellowjacket/backend/system"
 	"yellowjacket/backend/tagwriter"
 )
 
@@ -104,6 +107,17 @@ func NewYellowJacketApp(
 
 	yjApp.assetHandler.RegisterHandler(coverart.PathPrefix, coverHandler)
 
+	// Register artist image handler for serving cached artist photos.
+	artistImgDir, err := system.GetUserDataDirPath()
+	if err == nil {
+		artistImgHandler := http.StripPrefix(
+			"/artist-images/",
+			http.FileServer(http.Dir(filepath.Join(artistImgDir, "artist-images"))),
+		)
+
+		yjApp.assetHandler.RegisterHandler("/artist-images/", artistImgHandler)
+	}
+
 	// create playlist service
 	yjApp.playlist = playlist.NewService(
 		yjApp.logger, yjApp.database, yjApp.appConfig,