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

Cargo.lock

@@ -1916,7 +1916,7 @@ dependencies = [
 
 [[package]]
 name = "linux-patch-api"
-version = "1.1.17"
+version = "1.2.0"
 dependencies = [
  "actix",
  "actix-rt",
@@ -1942,10 +1942,12 @@ dependencies = [
  "serde_json",
  "serde_yaml",
  "serial_test",
+ "socket2 0.5.10",
  "sysinfo",
  "systemd",
  "tempfile",
  "thiserror 1.0.69",
+ "time",
  "tokio",
  "tokio-rustls",
  "tokio-test",