Embedded MQTT broker for ESP32 proxy

[KristianP26]

Summary

Add an optional embedded MQTT broker (e.g. aedes) so users of the ESP32 BLE proxy don't need to install and configure a separate broker like Mosquitto.

Motivation

The ESP32 proxy currently requires an external MQTT broker, which adds setup complexity for users who don't already have one (e.g. no Home Assistant). Embedding a lightweight broker directly in BLE Scale Sync would make the proxy a zero-config experience.

Proposed behavior

• If mqtt_proxy.broker_url is not set, start an embedded MQTT broker on mqtt://0.0.0.0:1883 and connect to it internally
• If mqtt_proxy.broker_url is set, use the external broker as before (no change)
• The ESP32 config.json would point to the server's IP on port 1883

Implementation notes

• aedes is a lightweight, well-maintained MQTT broker for Node.js
• The broker should start before the mqtt-proxy handler connects
• Port should be configurable (e.g. mqtt_proxy.embedded_broker_port)
• Docker: no extra container needed, the app container handles everything
• Existing users with external brokers are unaffected