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-web/src/routes/hosts.rs

@@ -112,6 +112,7 @@ async fn list_hosts(
             SELECT h.id, h.fqdn, host(h.ip_address)::text AS ip_address, h.display_name,
                    h.os_family, h.os_name, h.health_status, h.agent_version,
                    COALESCE(hpd.patch_count, 0) AS patches_missing,
+                   " + hc_subquery + ",
                    h.registered_at
             FROM hosts h
             LEFT JOIN host_patch_data hpd ON hpd.host_id = h.id
@@ -130,6 +131,7 @@ async fn list_hosts(
                    h.display_name, h.os_family, h.os_name,
                    h.health_status, h.agent_version,
                    COALESCE(hpd.patch_count, 0) AS patches_missing,
+                   " + hc_subquery + ",
                    h.registered_at
             FROM hosts h
             LEFT JOIN host_patch_data hpd ON hpd.host_id = h.id