Use print() for cookie debug log so it shows in container logs

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

backend/services/ytdlp.py

@@ -464,6 +464,8 @@ def start_download(
 
     def _run_download():
         with _SEMAPHORE:
+            cookie_args = _cookie_args()
+            print(f"[ytdlp] cookie_args={cookie_args!r} file_exists={Path(cookie_args[1]).exists() if len(cookie_args) == 2 and cookie_args[0] == '--cookies' else 'n/a'}", flush=True)
             process = subprocess.Popen(
                 [
                     "yt-dlp", url,