Publish Lap Events to MQTT

[bracyw]

Description

When a lap event occurs, the backend should publish to an MQTT topic so that subscribers like Nero can receive lap updates in real time. This enables external systems to react to lap changes as they happen.

Acceptance Criteria

• Lap start events are published to the appropriate MQTT topic with epoch microsecond timestamp
• External subscribers (e.g. Nero) can receive and process these events

Proposed Solution

Publish to timers/lap/<n> on the MQTT broker on each lap event. Reference existing MQTT publish patterns in scylla-server/src/. Test by subscribing with an MQTT client and verifying messages arrive correctly.

Mocks

No response