Add gracefulShutdown(), registerSignalHandlers(), and isShuttingDown()
to enable clean process termination during scraper operations.
Key changes:
- Register SIGTERM and SIGINT signal handlers for process shutdown
- Wait for in-progress scraper jobs to complete before exiting, with
a configurable timeout (SCRAPER_SHUTDOWN_TIMEOUT, default 30s)
- Release mutex lock on shutdown to prevent stale lock state
- Track shutdown state to reject new job acquisitions during shutdown
- Stop cron scheduler as part of the shutdown sequence
- Add SHUTDOWN_TIMEOUT to scraper config module
Adds 14 new tests covering shutdown lifecycle, timeout enforcement,
idempotent shutdown calls, and lock rejection during shutdown (56 total).
