优化客户端封面获取逻辑

修改emby客户端获取封面逻辑，修复没有设置封面导致封面为空的问题

internal/service/emby_compat.go

@@ -62,6 +62,9 @@ type EmbyService struct {
 
 	cloudProbeMu       sync.Mutex
 	cloudProbeInFlight map[string]struct{}
+
+	libraryCoverMu    sync.Mutex
+	libraryCoverCache map[string]embyArtworkCacheEntry
 }
 
 type cloudPlaybackResolver interface {