Drop redundant import sys inside _auto_download_disabled

Ruff F811 — `sys` is already imported at module top (line 13). The
local `import sys` inside `_auto_download_disabled` shadowed it
needlessly. Caught by CI ruff check on the dev-nightly workflow.

core/youtube_client.py

@@ -257,7 +257,6 @@ class YouTubeClient(DownloadSourcePlugin):
           — which is exactly when registry.py probes is_configured at
           web_server import)
         """
-        import sys
         return bool(
             os.environ.get('SOULSYNC_NO_FFMPEG_DOWNLOAD')
             or os.environ.get('PYTEST_CURRENT_TEST')