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/components/ZoneCard.tsx

@@ -2,7 +2,7 @@ import { Thermometer, BatteryLow, TriangleAlert, Snowflake, Flame, Fan, Droplet,
 import type { ZoneStatus } from '../types'
 import { ROOM_STATE_LABELS, MHI_FAN_SPEEDS } from '../types'
 
-const AC_DEVICE_TYPES = ['mhi_modbus', 'midea', 'daikin', 'home_assistant']
+const AC_DEVICE_TYPES = ['mhi_modbus', 'mhi_mqtt', 'midea', 'daikin', 'home_assistant']
 
 const MODE_ICONS: Record<string, typeof Snowflake> = {
   cool: Snowflake,

frontend/src/components/ZoneDetailModal.tsx

@@ -100,7 +100,7 @@ export default function ZoneDetailModal({ zone, onClose, onSaved }: {
               </div>
               <span className={`badge badge-health-${d.health_state}`}>{d.health_state.replace('_', ' ')}</span>
             </div>
-            {d.device_type === 'mhi_modbus' && <MhiControlPanel deviceId={d.id} canControl={canControl} />}
+            {(d.device_type === 'mhi_modbus' || d.device_type === 'mhi_mqtt') && <MhiControlPanel deviceId={d.id} canControl={canControl} />}
           </div>
         ))}