Debug: log fetch_video_metadata cookie args and errors

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/services/ytdlp.py

@@ -211,16 +211,19 @@ def _best_thumbnail(thumbnails: list | None) -> str | None:
 def fetch_video_metadata(video_id: str) -> dict | None:
     """Fetch metadata for a single video by YouTube ID."""
     url = f"https://www.youtube.com/watch?v={video_id}"
-    stdout, _, code = _run([
+    cookie_args = _cookie_args()
+    print(f"[fetch_meta] video={video_id} cookie_args={cookie_args!r}", flush=True)
+    stdout, stderr, code = _run([
         "yt-dlp",
         url,
         "--dump-json",
         "--no-download",
         "--no-playlist",
-        "--quiet",
         "--extractor-args", "youtube:player_client=ios,web",
-        *_cookie_args(),
+        *cookie_args,
     ], timeout=30)
+    if code != 0:
+        print(f"[fetch_meta] FAILED code={code} stderr={stderr[:500]!r}", flush=True)
 
     for line in stdout.splitlines():
         line = line.strip()