hvac: native MQTT driver for MHI aircon (replaces Modbus for control/status)

The Intesis MHI gateway was switched from Modbus TCP to native MQTT mode
(2026-08-13), publishing retained per-signal JSON status to
hvac/mhi/IU<nn>/status/<signal> and subscribing to commands at
hvac/mhi/IU<nn>/cmd/<signal>Cmd. This adds an MQTT ingest + command path
alongside the now-dormant Modbus driver, under a new mhi_mqtt device type.

Backend:
- lib/mqtt.js: subscribe hvac/mhi/+/status/+, parse JSON payloads, auto-
  register each unit as an unassigned mhi_mqtt zone_devices row and cache
  live status (power/mode/setpoint/room temp/fan/health). Add
  publishMhiCommand() for on/off/mode/setpoint/fan/vanes writes (QoS 1,
  retain off — a retained command would replay stale on gateway reconnect).
  Numeric mode/fan enums map index-for-index to the frontend label arrays.
- routes/override.js: mhi-status/mhi-control branch on device_type —
  mhi_mqtt serves from the push-populated cache and publishes commands;
  mhi_modbus keeps its synchronous register path unchanged.
- routes/devices.js: discover() note for mhi_mqtt (units self-register).
- No DB migration: existing status columns cover it.

Credentials: getCredentials() now prefers env-injected creds
(MQTT_BROKER_HOST/USERNAME/PASSWORD) over the settings fetch — the broker
hashes each dynsec client password show-once, so it can't be re-served from
settings at runtime. Compose passes the new env through.

Frontend: add mhi_mqtt to DeviceType/labels/implemented list, render
MhiControlPanel and count it as an AC device type.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/pages/Devices.tsx

@@ -10,7 +10,7 @@ import GatewaysPanel from '../components/GatewaysPanel'
 import { useAuth } from '../components/AuthGate'
 import { can } from '../types'
 
-const ALL_DEVICE_TYPES: DeviceType[] = ['shelly_trv', 'mhi_modbus', 'midea', 'daikin', 'home_assistant']
+const ALL_DEVICE_TYPES: DeviceType[] = ['shelly_trv', 'mhi_mqtt', 'mhi_modbus', 'midea', 'daikin', 'home_assistant']
 
 export default function Devices() {
   const { user } = useAuth()