Add Server-Sent Events for Real-Time Updates

## Summary

SSE endpoint for real-time fault notifications.

---

## Proposed solution (optional)

FaultManager publishes:
```
/fault_manager/events (FaultEvent.msg)
```

Gateway exposes:
```
GET /api/v1/faults/stream
```
Events: fault_confirmed, fault_cleared, fault_updated

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## Additional context (optional)

- Handle multiple clients
- Keepalive every 30s
- Include test HTML page