Add MQTT reconnect backoff

[TalkingJupiter]

Context

The current MQTT reconnect helper retries whenever publish/connect logic reaches it. If the broker is unavailable, this can cause overly aggressive reconnect attempts.

Scope

Add bounded reconnect backoff to the MQTT sender.

Suggested behavior

• Track the last reconnect attempt timestamp.
• Only retry every 2-5 seconds while disconnected.
• Keep mqttClient.loop() running when connected.
• Do not block heartbeat or temperature sampling.

Acceptance criteria

• Broker-down scenarios do not spam reconnect attempts every main loop.
• Serial logs remain useful but not noisy.
• Telemetry resumes automatically when broker connectivity returns.
• A/B heartbeat and failover logic remain responsive while MQTT is down.