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

crates/pm-agent-client/src/lib.rs

@@ -39,5 +39,5 @@ pub use error::AgentClientError;
 /// Response envelope and all data types.
 pub use types::{
     AgentEnvelope, AgentErrorBody, HealthData, Package, PackagesData, Patch, PatchesData,
-    SystemInfoData,
+    RollbackResponse, ServiceStatusData, SystemInfoData,
 };