Add MHI aircon manual control (Phase 2, Modbus only)

Manual on/off/mode/setpoint/fan control for MHI indoor units via the
already-verified Intesis Modbus TCP gateway, deliberately NOT wired into
the NewBook-driven scheduler yet — that stays deferred. Modbus doesn't
depend on the MQTT broker (separate infra, still unbuilt), so this can be
deployed and tested standalone.

- mhi_gateways table (connection config: host/port/slave id/address base)
  + zone_devices columns for MHI devices (gateway id, unit index, IU hint,
  and the per-device register map staged from a MAPS import — ground
  truth, never re-derived from room/IU at runtime)
- drivers/mhi-modbus.js: discover/getStatus/setTarget over modbus-serial,
  same interface as trv.js/homeassistant.js; per-gateway request queue
  since Modbus TCP requires serialised requests; falls back to the
  profile's stride formula (with a loud warning) only if a unit has never
  been through an import
- routes/mhi-gateways.js: gateway CRUD, live test-connection, xlsx import
  (stages the parsed result for review — never auto-creates devices),
  per-unit 'assign to zone' as the explicit commit step
- routes/override.js: GET .../mhi-status, POST .../mhi-control (control cap)
- frontend: GatewaysPanel (add gateway, test connection, import + preview,
  assign units) and MhiControlPanel (on/off, mode, setpoint, fan) wired
  into Devices.tsx / ZoneDetailModal.tsx

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/src/routes/override.js

@@ -1,6 +1,7 @@
 import { requireAuth, requireCap } from '../auth.js'
 import { pool, logActivity } from '../db.js'
 import { batchSetZoneTemperature } from '../lib/mqtt.js'
+import * as mhiDriver from '../lib/drivers/mhi-modbus.js'
 
 // Manual force-temperature — the equivalent of the old integration's
 // newbook.force_room_temperature service. Disables auto mode for the zone so the
@@ -38,4 +39,49 @@ export async function overrideRoutes(app) {
 
     return { ok: successful > 0, successful, total: devices.length, auto_mode: false }
   })
+
+  // GET /api/devices/:id/mhi-status — live Modbus read (onOff/mode/setpoint/
+  // fanSpeed/roomTemp/errorCode/etc) for one assigned MHI aircon unit.
+  app.get('/api/devices/:id/mhi-status', { preHandler: requireCap('view') }, async (req, reply) => {
+    const { rows } = await pool.query(
+      `SELECT * FROM zone_devices WHERE id = $1 AND device_type = 'mhi_modbus'`,
+      [req.params.id]
+    )
+    if (!rows.length) return reply.status(404).send({ error: 'MHI device not found' })
+    const status = await mhiDriver.getStatus(rows[0].external_ref)
+    if (!status || status.error) {
+      return reply.status(502).send({ error: status?.error || 'Failed to read device status' })
+    }
+    return status
+  })
+
+  // POST /api/devices/:id/mhi-control — manual aircon control only (no
+  // scheduler wiring). Body: { onOff?, mode?, tempC?, fanSpeed? } — only the
+  // fields present are written, via the mhi-modbus driver's setTarget().
+  app.post('/api/devices/:id/mhi-control', { preHandler: requireCap('control') }, async (req, reply) => {
+    const { rows } = await pool.query(
+      `SELECT * FROM zone_devices WHERE id = $1 AND device_type = 'mhi_modbus'`,
+      [req.params.id]
+    )
+    if (!rows.length) return reply.status(404).send({ error: 'MHI device not found' })
+    const device = rows[0]
+    const { onOff, mode, tempC, fanSpeed } = req.body || {}
+
+    const ok = await mhiDriver.setTarget(device.external_ref, { onOff, mode, tempC, fanSpeed })
+
+    if (ok && tempC !== undefined && tempC !== null) {
+      await pool.query(
+        `UPDATE zone_devices SET current_target_temp = $1, target_origin = 'manual', updated_at = NOW() WHERE id = $2`,
+        [tempC, device.id]
+      )
+    }
+
+    await logActivity(device.zone_id, 'device_command', {
+      note: `MHI manual control on ${device.location || device.discovered_name || device.external_ref}: ${JSON.stringify({ onOff, mode, tempC, fanSpeed })}${ok ? '' : ' — FAILED'}`,
+      source: 'manual',
+      userEmail: req.user.email,
+    })
+
+    return { ok }
+  })
 }