fix: use iOS player client to bypass YouTube bot detection

YouTube restricts available formats for bot-like requests. Specifying
player_client=ios,web makes yt-dlp impersonate the iOS app which has
broader format access without requiring cookies.

app/downloader.py

@@ -49,6 +49,7 @@ async def extract_audio(url: str) -> Path:
         "--audio-format", "mp3",
         "--audio-quality", "128K",
         "--format", "bestaudio/best/b",
+        "--extractor-args", "youtube:player_client=ios,web",
         "--output", outtmpl,
     ]