feat: add graceful shutdown handling to scraper job scheduler

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).

config/scraper.js

@@ -27,6 +27,9 @@ module.exports = {
     timeout: parseInt(process.env.SCRAPER_TIMEOUT, 10) || 30000
   },
 
+  // Graceful shutdown timeout (how long to wait for running job before force-stopping)
+  SHUTDOWN_TIMEOUT: parseInt(process.env.SCRAPER_SHUTDOWN_TIMEOUT, 10) || 30000,
+
   // MongoDB collection names (environment variable overrides for development isolation)
   COLLECTIONS: {
     UNITS: process.env.SCRAPER_UNITS_COLLECTION || 'units_migration_test',