feat(events): backend SSE /api/events + event broker (part 1 of #93)

[umi-appcoder]

What

Adds the backend push infrastructure for #93 — an in-process event broker and a GET /api/events SSE stream — so the SPA can replace per-route polling with real-time updates. Part 1 of 2 (backend); Part 2 wires the SPA client and drops the poll loops.

Built directly on the #96 reconcile loop (just merged): the background reconciler now also emits events, so updates flow even when no client is reading a task.

Pieces

• EventBroker — pub/sub fan-out to connected SSE clients. Bounded per-subscriber queues; a slow client drops its oldest event rather than blocking the publisher (SSE is lossy-tolerant — the SPA reconciles via a normal fetch on reconnect). publish() never raises and never blocks the caller.
• GET /api/events — SSE firehose: subscribes to the broker, forwards each event as a named frame (event: task.status\ndata: {...}), heartbeats to keep proxies from closing idle connections, and caps lifetime via STREAM_MAX_SECONDS (client reconnects on the end event). Framing mirrors the existing handle_claude_stream_output (incl. X-Accel-Buffering: no).
• Emit points — fire on real transitions, so any caller (the background reconciler or a lazy list/get) triggers them:
  • task.created — create_task / create_terminal_task
  • task.status — _reconcile_status completed / waiting-for-input / back-to-running

Tests

tests/event_broker_test.py (8): fan-out, unsubscribe, no-subscriber safety, slow-consumer drop-oldest, default-empty-data, and the reconcile → task.status integration. Full Python suite: 466, OK.

Scope / next

Part 2 (separate PR): SPA EventSource client → fan events into the existing stores → drop the setInterval poll loops (keeping a low-frequency poll fallback when the stream drops). That also completes part 2 of #101.

Future emit types noted in #93 (trigger.fired, memory.changed) can be added incrementally on the same broker.

Part 1 of #93.

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