feat: implement graceful shutdown and enhanced job recovery

- Add comprehensive graceful shutdown manager with signal handling
- Implement container-aware shutdown with proper signal forwarding
- Add shutdown-aware job processing with automatic state persistence
- Enhance cleanup service with proper shutdown coordination
- Add integration tests for graceful shutdown functionality
- Update Docker entrypoint for proper signal handling
- Add comprehensive documentation for shutdown process

Features:
- Fast shutdown (under 30 seconds) without waiting for job completion
- Automatic job state saving and recovery after restart
- Support for SIGTERM, SIGINT, SIGHUP signals
- Container orchestrator compatibility (Docker, Kubernetes)
- Zero data loss during container lifecycle events
- Detailed logging and monitoring capabilities

Version: 2.9.0

package.json

@@ -1,7 +1,7 @@
 {
   "name": "gitea-mirror",
   "type": "module",
-  "version": "2.8.0",
+  "version": "2.9.0",
   "engines": {
     "bun": ">=1.2.9"
   },