Problem: each scraper defined its own timeout constants
(`TIMEOUT_S`, `TIMEOUT_SECONDS`) with inconsistent values (15s vs 30s)
and browser timeouts were scattered as magic numbers (60000, 15000,
5000, 500).
Solution: introduce `scrapers/timeouts.py` with named constants for
HTTP requests, browser session/navigation/element/turnstile/settle
timeouts, and submission polling. All six scrapers now import from
the shared module.
