Playwright's sync API binds its greenlet event loop to the creating thread.
Sharing pre-warmed slots across threads caused "cannot switch to a different
thread" panics under uvicorn. New design: each worker thread owns its own
Playwright instance created lazily on first fetch_html() call. A registry
dict keyed by thread-id lets stop() close all slots at shutdown. Removes
ThreadPoolExecutor warmup and idle-cleanup daemon thread entirely.
