feat: add auto-enrollment, cert validation, and crash loop fixes

- Auto-enrollment on startup when certs are missing/invalid and enrollment.manager_url configured
- Certificate validation (existence, parse, expiry, key match, CA trust)
- --enroll exits after completion (no port conflict with systemd service)
- --renew-certs flag for manual cert renewal
- SO_REUSEADDR on TcpListener::bind (prevents Address already in use)
- Polling token persistence for enrollment resume after restart
- Exit code strategy (0=clean, 1=error, 2=enrollment in progress)
- HTTP 409 (host already exists) handling during enrollment
- Move 'Listening on' log after actual bind
- Increase RestartSec to 10s and add StartLimitBurst=5
- Postinst checks for certs and enrollment URL, prints guidance
- EnrollmentConfig.manager_url changed to Option<String>
- cert_renewal_threshold_days and polling_token config fields
- Updated SPEC.md and DEPLOYMENT_GUIDE.md with new workflow
- RCA document for crash loop root cause analysis
- Version bumped to 1.2.0

configs/linux-patch-api.service

@@ -9,7 +9,9 @@ Type=simple
 NotifyAccess=all
 ExecStart=/usr/bin/linux-patch-api --config /etc/linux_patch_api/config.yaml
 Restart=on-failure
-RestartSec=5s
+RestartSec=10s
+StartLimitBurst=5
+StartLimitIntervalSec=300
 TimeoutStopSec=30s
 
 # Process management