Issue: Add MQTT health‑check endpoint

Issue: Add MQTT health‑check endpoint: a task in LegoFlow-SWE (Harbor dataset). Problem There is currently no way to verify that MQTT messaging is operational without sending messages on real data topics. A dedicated health‑check mechanism is needed so that clients can test connectivity without…

The task

**Problem** There is currently no way to verify that MQTT messaging is operational without sending messages on real data topics. A dedicated health‑check mechanism is needed so that clients can test connectivity without affecting production data or requiring access to any specific channel.

Part of Lego-X/LegoFlow-SWE.