Update bundled yt-dlp and report source failures in chat

The image shipped a year-old yt-dlp, which YouTube now rejects with
"The page needs to be reloaded". Bumped to 2026.08.19 and documented
rebuilding as the standard fix, including how to pass a newer tag
without waiting for a repository update.

A downloader dying mid-stream also looked exactly like a very short
track: ffmpeg saw EOF, the player advanced, and the channel only got
"queue finished". The failure reason now reaches the chat.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/sources/index.ts

@@ -110,6 +110,8 @@ export interface PlaybackInput {
   input: string | Readable;
   inputOptions: string[];
   cleanup(): void;
+  /** Resolves with a reason if the downloader died on its own, for reporting. */
+  failure?: Promise<string | null>;
 }
 
 const HTTP_RESILIENCE = [
@@ -139,5 +141,10 @@ export async function openPlayback(track: Track, seekSeconds = 0): Promise<Playb
   }
 
   const proc = ytdlp.openAudioStream(track.url);
-  return { input: proc.stream, inputOptions: [], cleanup: () => proc.kill() };
+  return {
+    input: proc.stream,
+    inputOptions: [],
+    cleanup: () => proc.kill(),
+    failure: proc.failure,
+  };
 }