fix: Improve Docker container shutdown handling

- Add proper signal handlers for graceful shutdown of all components
- Implement WebhookBatcher.shutdown() to properly close ThreadPoolExecutor and cancel timers
- Add NFOGuardDatabase.close() method to close thread-local database connections
- Increase uvicorn graceful shutdown timeout to 15 seconds
- Add cleanup in finally block to ensure resources are freed even if signal handler fails
- Reduce logging and server header overhead for better performance

This should resolve Docker container shutdown issues where containers don't respond to SIGTERM properly.

Bump version to 2.1.1

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VERSION

@@ -1 +1 @@
-2.1.0
+2.1.1