Add MQTT hello/bootstrap publish

[TalkingJupiter]

Context

Before the Radxa can send per-device configuration, each ESP32 should announce itself to the broker after connecting.

Scope

Add a hello/bootstrap message to the MQTT sender.

Suggested topic:

repacss/devices/<mac>/hello

Suggested payload fields:

• message_type: hello
• mac: full device MAC
• client_id: generated MQTT client ID
• source: firmware / ethernet
• optional firmware build metadata if available later

Implementation notes

• Build the hello topic beside gTelemetryTopic in TelemetrySender.cpp.
• Publish hello after connectMqtt() succeeds.
• Consider retained vs non-retained behavior. Retained may help the Radxa know the latest connected devices, but choose deliberately.

Acceptance criteria

• ESP32 publishes a hello message after MQTT connect.
• Broker subscribers to repacss/# can see the hello payload.
• Hello publish failure is logged without crashing telemetry.