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

config/config.example.toml

@@ -46,7 +46,11 @@ patch_poll_interval_secs = 1800
 max_concurrent_agent_calls = 64
 
 # Worker heartbeat write interval (seconds)
-heartbeat_interval_secs = 30
+
+# WS relay HTTP polling fallback interval (seconds). When WebSocket connection to
+# an agent fails, the relay falls back to polling the agent's HTTP API at this
+# interval. Default: 10
+ws_relay_poll_interval_secs = 10
 
 # ============================================================
 # Logging