Add MQTT config subscription and callback handling

[TalkingJupiter]

Context

RuntimeConfig parsing exists, but the firmware does not yet subscribe to any MQTT config topic or apply received config.

Scope

Subscribe to a per-device config topic and parse config messages.

Suggested topic:

repacss/devices/<mac>/config

Implementation notes

• Add mqttClient.setCallback(...) in TelemetrySender.cpp.
• Subscribe after successful MQTT connect.
• Convert the callback payload into a String or bounded buffer safely.
• Pass config JSON into parseRuntimeConfigJson(...).
• Store accepted config somewhere the main firmware can use.

Acceptance criteria

• Firmware subscribes to the config topic after MQTT connect/reconnect.
• Retained config messages are received after reconnect.
• Valid config JSON is parsed into RuntimeConfig.
• Invalid config JSON is rejected with a useful serial log message.
• Telemetry publishing continues even if config is invalid or missing.