feat(integrations): Home Assistant MQTT discovery preset

[MarkCesium]

Summary

Home Assistant is the dominant DIY IoT hub. Supporting its MQTT discovery protocol makes PlaceBrain useful out-of-the-box for every existing HA user, without them re-wiring devices.

Changes

• Toggle on device creation ("Publish Home Assistant discovery").
• When enabled, on device/sensor creation the backend (gateway or devices) publishes retained messages to homeassistant/<component>/<node_id>/<object_id>/config per HA's schema.
• Schema covers sensors, binary_sensors, switches, number/actuator types.
• Docs section in feat(docs): in-repo user documentation site #19.

Verification

• Point a local HA instance at the same MQTT broker.
• Create a device with discovery enabled — it appears in HA under MQTT integration with correct device class and unit.