trafilatura.extract dispatched via run_in_executor isn't safe to run
concurrently — two parallel calls can crash the process with a
libxml2-level double free. The top-level Wikipedia+SearXNG gather is
fine; only the inner per-article extraction needs to stay sequential,
matching the pre-parallelization behavior.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>