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-core/src/audit.rs

@@ -47,6 +47,9 @@ pub enum AuditAction {
     PatchJobCompleted,
     PatchJobFailed,
     MaintenanceWindowReminder,
+    HealthCheckCreated,
+    HealthCheckUpdated,
+    HealthCheckDeleted,
 }
 
 impl AuditAction {
@@ -80,6 +83,9 @@ impl AuditAction {
             Self::PatchJobCompleted => "patch_job_completed",
             Self::PatchJobFailed => "patch_job_failed",
             Self::MaintenanceWindowReminder => "maintenance_window_reminder",
+            Self::HealthCheckCreated => "health_check_created",
+            Self::HealthCheckUpdated => "health_check_updated",
+            Self::HealthCheckDeleted => "health_check_deleted",
         }
     }
 }