feat: health check configuration and worker engine (Phase 3+4)

- Added health_check_poller.rs: periodic service/HTTP health checks
- Added pre-patch health gate in job_executor.rs
- Added waiting_health_check job status (migration 008)
- Added health_check_status to HostSummary and hosts API
- Added health check types and API functions to frontend
- Added health check UI section to HostDetailPage
- Added health check status indicators to HostsPage and PatchDeploymentPage
- Added serde default for health_check_poll_interval_secs
- Fixed missing AgentClient import in health_check_poller.rs
- Fixed missing ws_relay import in main.rs
- Fixed missing closing paren in retry_pending_jobs SQL
- Added ReadWritePaths for /etc/patch-manager/keys in systemd services

Cargo.toml

@@ -77,5 +77,6 @@ totp-rs = { version = "5", features = ["gen_secret", "otpauth"] }
 base64 = { version = "0.22" }
 hex = { version = "0.4" }
 sha2 = { version = "0.10" }
+aes-gcm = { version = "0.10" }
 ipnet = { version = "2" }
 url = { version = "2" }