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

.gitignore

@@ -1,2 +1,15 @@
 /target
 /releases/
+
+# Build artifacts
+debian/tmp/
+debian/linux-patch-api/
+debian/.debhelper/
+debian/debhelper-build-stamp
+debian/files
+debian/linux-patch-api.debhelper.log
+debian/linux-patch-api.postrm.debhelper
+debian/linux-patch-api.substvars
+*.deb
+*.buildinfo
+*.changes