Fetch metadata when something is added to the playlist.

frontend/src/api/player.tsx

@@ -7,11 +7,26 @@ export interface NowPlayingResponse {
   currentFile: string;
 }
 
+export interface Metadata {
+  title?: string;
+  description?: string;
+  siteName?: string;
+}
+
 export interface PlaylistItem {
   filename: string;
   title: string | null;
   id: number;
   playing: boolean | null;
+  metadata?: Metadata;
+}
+
+export interface MetadataUpdateEvent {
+    event: 'metadata_update';
+    data: {
+        url: string;
+        metadata: Metadata;
+    };
 }
 
 export const API = {