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/lib.rs

@@ -1,5 +1,6 @@
 pub mod audit;
 pub mod config;
+pub mod crypto;
 pub mod db;
 pub mod error;
 pub mod logging;
@@ -8,11 +9,14 @@ pub mod request_id;
 
 // Re-export commonly used types
 pub use config::AppConfig;
+pub use crypto::{CryptoError, KEY_PATH, decrypt, encrypt, load_or_create_key};
 pub use error::{AppError, ErrorResponse};
 pub use models::{
-    AuthProvider, CreateGroupRequest, CreateHostRequest, CreateUserRequest, DiscoveryCidrRequest,
-    DiscoveryResult, Group, Host, HostHealthStatus, HostSummary, RegisterDiscoveredRequest,
-    UpdateGroupRequest, UpdateUserRequest, User, UserRole as DbUserRole,
+    AuthProvider, CreateGroupRequest, CreateHealthCheckRequest, CreateHostRequest,
+    CreateUserRequest, DiscoveryCidrRequest, DiscoveryResult, Group, HealthCheck,
+    HealthCheckResult, HealthCheckWithResult, Host, HostHealthStatus, HostSummary,
+    RegisterDiscoveredRequest, UpdateGroupRequest, UpdateHealthCheckRequest, UpdateUserRequest,
+    User, UserRole as DbUserRole,
 };
 
 // Re-export audit integrity types