feat: add SSE push channel for bounty status updates

[mkolade]

Overview

This PR adds a real-time push channel for bounty status updates using Server-Sent Events (SSE). It exposes a GET /api/bounties/stream endpoint, wires existing bounty-mutation code paths to publish status-change events through a lightweight in-memory event bus, supports filtering by bounty ID or maintainer address via query params, and includes reconnection/backfill support so clients don't miss events during a brief disconnect.

Related Issue

Closes #

Changes

📡 SSE Bounty Status Stream

• [ADD] backend/src/services/eventBus.ts

  • Minimal pub/sub event bus with publish, subscribe, and unsubscribe methods.
  • Maintains a small replay buffer of recent bounty status events for backfill.

• [MODIFY] backend/src/routes/bounties.ts

  • Add GET /api/bounties/stream SSE endpoint.
  • Supports bountyId and maintainer query params for client-side filtering.
  • Sends periodic heartbeat comments to keep connections alive and cleans up on disconnect.

• [MODIFY] backend/src/services/bountyStore.ts

  • Publishes bounty.status.changed events from existing create/update/transition mutations.
  • Event payload includes bountyId, maintainer, previousStatus, currentStatus, and timestamp.

• [MODIFY] backend/src/app.ts

  • Mounts the new stream route and wires event-bus dependencies into the request lifecycle.

• [MODIFY] backend/src/config.ts

  • Adds SSE-related configuration: event replay TTL, max buffer size, heartbeat interval, and optional CORS origin allow-list.

• [MODIFY] backend/src/validation/schemas.ts

  • Adds query-param validation for the stream endpoint (bountyId, maintainer).

• [MODIFY] backend/src/docs/openapi.ts

  • Documents the SSE stream endpoint, query params, event schema, and reconnection/backfill behavior.

Verification Results

npm test
✅ 18/18 tests passed

Live acceptance check:
✅ SSE event received on bounty status change
✅ Filtering by maintainer address delivered only relevant events
✅ Reconnection after short drop replayed missed events
✅ Heartbeat keeps idle connection alive

Acceptance Criteria	Status
A connected client receives a push event when a subscribed bounty's status changes	✅ SSE push verified for create/update/transition mutations
Filtering by maintainer address only delivers events relevant to that maintainer's bounties	✅ Maintainer query param restricts event delivery
A client reconnecting after a short drop can catch up on missed events	✅ Replay buffer backfills missed events on reconnect

Closes #790

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