Log cookie args on download start for debugging

backend/services/ytdlp.py

@@ -464,6 +464,9 @@ def start_download(
 
     def _run_download():
         with _SEMAPHORE:
+            cookie_args = _cookie_args()
+            import logging
+            logging.getLogger(__name__).info("download cookie_args=%r file_exists=%s", cookie_args, Path(cookie_args[1]).exists() if len(cookie_args) == 2 and cookie_args[0] == "--cookies" else "n/a")
             process = subprocess.Popen(
                 [
                     "yt-dlp", url,