fix(artwork): avoid data race in cache warmer config access

Capture UICoverArtSize at construction time instead of reading from
conf.Server on each doCacheImage call. The background goroutine could
race with test config cleanup, causing intermittent race detector
failures in CI.

core/artwork/cache_warmer.go

@@ -37,10 +37,11 @@ func NewCacheWarmer(artwork Artwork, cache cache.FileCache) CacheWarmer {
 	}
 
 	a := &cacheWarmer{
-		artwork:    artwork,
-		cache:      cache,
-		buffer:     make(map[model.ArtworkID]struct{}),
-		wakeSignal: make(chan struct{}, 1),
+		artwork:      artwork,
+		cache:        cache,
+		buffer:       make(map[model.ArtworkID]struct{}),
+		wakeSignal:   make(chan struct{}, 1),
+		coverArtSize: conf.Server.UICoverArtSize,
 	}
 
 	// Create a context with a fake admin user, to be able to pre-cache Playlist CoverArts
@@ -50,11 +51,12 @@ func NewCacheWarmer(artwork Artwork, cache cache.FileCache) CacheWarmer {
 }
 
 type cacheWarmer struct {
-	artwork    Artwork
-	buffer     map[model.ArtworkID]struct{}
-	mutex      sync.Mutex
-	cache      cache.FileCache
-	wakeSignal chan struct{}
+	artwork      Artwork
+	buffer       map[model.ArtworkID]struct{}
+	mutex        sync.Mutex
+	cache        cache.FileCache
+	wakeSignal   chan struct{}
+	coverArtSize int
 }
 
 func (a *cacheWarmer) PreCache(artID model.ArtworkID) {
@@ -141,7 +143,7 @@ func (a *cacheWarmer) doCacheImage(ctx context.Context, id model.ArtworkID) erro
 	ctx, cancel := context.WithTimeout(ctx, 10*time.Second)
 	defer cancel()
 
-	size := conf.Server.UICoverArtSize
+	size := a.coverArtSize
 	r, _, err := a.artwork.Get(ctx, id, size, true)
 	if err != nil {
 		return fmt.Errorf("caching id='%s', size=%d: %w", id, size, err)