Add a Server-Sent Events endpoint /xce/v1/events streaming job lifecycle
events (job.created/updated/deleted/finished) to subscribers. The
DefaultHandler now publishes every job event to a process-wide EventBroker;
the SSE handler subscribes and streams JSON frames with a keep-alive comment
during idle.
- backend/broker.go: thread-safe fan-out Broker (drops for slow consumers so
the notification path never blocks), Event type with job_status string.
- backend/handler.go: OnJob* publish to DefaultBroker.
- controllers/events.go: SSE handler (gin, text/event-stream, keep-alive).
- router + whitelist + rbac: /xce/v1/events route, event:subscribe
permission granted to mcp_readonly/mcp_operator/cluster_admin.
- Tests: broker fan-out/slow-consumer/unsubscribe, handler publishes, SSE
end-to-end via httptest.
Verified live on t1: SSE stream delivered job.updated PEND→RUN events for a
submitted job.
Co-Authored-By: Claude <noreply@anthropic.com>
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