Fix double _cookie_args() call in download; fix diagnostic player client

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

backend/services/ytdlp.py

@@ -556,7 +556,7 @@ 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)
+            print(f"[ytdlp] cookie_args={cookie_args!r}", flush=True)
             process = subprocess.Popen(
                 [
                     "yt-dlp", url,
@@ -568,7 +568,7 @@ def start_download(
                     "-o", output_template,
                     "--newline", "--progress", "--no-colors",
                     "--extractor-args", "youtube:player_client=web",
-                    *_cookie_args(),
+                    *cookie_args,
                 ],
                 stdout=subprocess.PIPE,
                 stderr=subprocess.STDOUT,