fix: add ALPN http/1.1 for WebSocket, polling fallback, and job-level WS events

- ws_relay.rs: Add ALPN protocol http/1.1 to rustls ClientConfig to prevent
  HTTP/2 negotiation which breaks WebSocket upgrades (Sec-WebSocket-Accept mismatch)
- ws_relay.rs: Add detailed TLS error chain logging for debugging connection failures
- ws_relay.rs: Add HTTP polling fallback when WebSocket connection fails, using
  AgentClient to poll /api/v1/jobs/{id} every ws_relay_poll_interval_secs
- config.rs: Add ws_relay_poll_interval_secs field (default: 10 seconds)
- config.example.toml: Add ws_relay_poll_interval_secs documentation
- jobs.rs: Fire pg_notify with event_type job on cancel
- job_executor.rs: Fire pg_notify with event_type job when parent job transitions
- ws_relay.rs: Add event_type field to NotifyPayload (host vs job events)
- Frontend: Add event_type, succeeded_count, failed_count, host_count to JobWsEvent
- Frontend: handleWsEvent distinguishes host vs job events for accurate status updates

crates/pm-core/src/config.rs

@@ -43,6 +43,8 @@ pub struct WorkerConfig {
     pub max_concurrent_agent_calls: usize,
     /// Worker heartbeat interval in seconds
     pub heartbeat_interval_secs: u64,
+    /// WS relay HTTP polling fallback interval in seconds (default: 10)
+    pub ws_relay_poll_interval_secs: u64,
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize)]
@@ -115,6 +117,7 @@ impl Default for AppConfig {
                 patch_poll_interval_secs: 1800,
                 max_concurrent_agent_calls: 64,
                 heartbeat_interval_secs: 30,
+                ws_relay_poll_interval_secs: 10,
             },
             logging: LoggingConfig {
                 level: "info".to_string(),