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/ytdlp.ts

@@ -209,6 +209,8 @@ export async function resolveStreamUrl(pageUrl: string): Promise<string> {
 export interface AudioProcess {
   stream: Readable;
   kill(): void;
+  /** Resolves with a reason when the download fails, or null when it was fine. */
+  failure: Promise<string | null>;
 }
 
 /** Spawns yt-dlp writing the best audio to stdout, for piping straight into ffmpeg. */
@@ -220,19 +222,29 @@ export function openAudioStream(pageUrl: string): AudioProcess {
   );
 
   let stderr = "";
+  let killed = false;
   child.stderr.setEncoding("utf8");
   child.stderr.on("data", (chunk: string) => {
     stderr = (stderr + chunk).slice(-2000);
   });
-  child.on("close", (code) => {
-    if (code !== 0 && code !== null && stderr.trim()) {
+
+  const failure = new Promise<string | null>((resolve) => {
+    child.on("close", (code) => {
+      if (killed || code === 0 || code === null) {
+        resolve(null);
+        return;
+      }
       log.warn({ code, stderr: stderr.slice(0, 500) }, "yt-dlp stream exited with error");
-    }
+      resolve(firstUsefulError(stderr));
+    });
+    child.on("error", (err: Error) => resolve(err.message));
   });
 
   return {
     stream: child.stdout,
+    failure,
     kill: () => {
+      killed = true;
       if (child.exitCode === null) child.kill("SIGKILL");
     },
   };