S3: Sensor-to-cloud pipeline is reliable and low-latency

[xishell]

Persona

Harbor master

User outcome

As a harbor master, I see live berth state in the dashboard within 5 seconds of the physical change, because the sensor-to-cloud pipeline is reliable and low-latency.

Covers everything between a sensor reading change and the backend database being updated: BLE Mesh transport, gateway uplink, MQTT broker, backend ingest, event logging.

Acceptance Criteria

• Gateway receives data from multiple sensor nodes simultaneously
• MQTT payloads match the R3 contract
• Gateway publishes a last-will message on unexpected disconnect
• Status topic uses retain=true so new subscribers get last known state
• End-to-end latency from sensor change to DB update < 5s
• Backend writes event rows on every state change