Add device telemetry history + per-device graph view

activity_log only ever recorded state transitions/overrides/sync/errors —
useful for "what did the app decide" but not "what was the room actually
doing" when a guest/staff complaint comes in after the fact.

Backend:
- New device_readings time-series table (room_temp, target_temp, mode,
  power_state, fan_speed, valve_position, heating_active), throttled to
  ~1 row/device/minute via recordReading() in db.js regardless of how
  often a caller fires — MQTT pushes can be far chattier than that.
- Wired into every place zone_devices' live-status cache already gets
  updated: ac-poller.js, daikin-poller.js, and mqtt.js's TRV
  (handleStatusBlob/handleInfo) and native-MQTT MHI (writeMhiState)
  handlers.
- 4-day retention by default (config key history_retention_days),
  pruned every 6h by the new history-pruner.js.
- GET /api/devices/:id/history?hours=96 to read it back.

Frontend:
- DeviceHistoryModal: room temp vs setpoint line chart, plus a
  valve-position chart (TRVs) or fan-speed step chart (AC), plus a
  diffed mode/power/fan change log — 24h/4d/7d range picker. Uses
  recharts, matching the convention already established in
  utilities/wages/reports.
- Graph icon on each device in both ZoneDetailModal and the Devices
  page opens it.

Side effect: frontend/package-lock.json was missing vite-plugin-pwa's
whole dependency tree even though package.json already declared it —
pre-existing drift, reconciled by this npm install (unrelated to the
recharts addition, just came along with it).

frontend/src/pages/Devices.tsx

@@ -1,5 +1,5 @@
 import { Fragment, useEffect, useState } from 'react'
-import { RadioTower, ChevronDown, ChevronRight, Wrench, Trash2 } from 'lucide-react'
+import { RadioTower, ChevronDown, ChevronRight, Wrench, Trash2, LineChart } from 'lucide-react'
 import {
   fetchDevices, discoverDevices, updateDevice, deleteDevice, fetchZones, fetchDevicePhotos, createMaintenanceAsset,
 } from '../api'
@@ -7,6 +7,7 @@ import type { Device, Zone, DevicePhoto, DeviceType } from '../types'
 import { DEVICE_TYPE_LABELS, IMPLEMENTED_DEVICE_TYPES } from '../types'
 import DevicePhotoUpload from '../components/DevicePhotoUpload'
 import GatewaysPanel from '../components/GatewaysPanel'
+import DeviceHistoryModal from '../components/DeviceHistoryModal'
 import { useAuth } from '../components/AuthGate'
 import { can } from '../types'
 
@@ -24,6 +25,7 @@ export default function Devices() {
   const [discovering, setDiscovering] = useState<DeviceType | null>(null)
   const [expanded, setExpanded] = useState<number | null>(null)
   const [photosByDevice, setPhotosByDevice] = useState<Record<number, DevicePhoto[]>>({})
+  const [historyDevice, setHistoryDevice] = useState<Device | null>(null)
 
   function load() {
     Promise.all([fetchDevices(), fetchZones()])
@@ -194,6 +196,9 @@ export default function Devices() {
                           </div>
                         </div>
                         <div style={{ display: 'flex', gap: 8, flexWrap: 'wrap', alignItems: 'center' }}>
+                          <button className="btn btn-sm" onClick={() => setHistoryDevice(d)}>
+                            <LineChart size={13} strokeWidth={1.75} /> View history
+                          </button>
                           {canManage && d.zone_id && (
                             <button className="btn btn-sm" onClick={() => createAsset(d.id)}>
                               <Wrench size={13} strokeWidth={1.75} /> Create asset in Maintenance
@@ -214,6 +219,8 @@ export default function Devices() {
           </table>
         </div>
       )}
+
+      {historyDevice && <DeviceHistoryModal device={historyDevice} onClose={() => setHistoryDevice(null)} />}
     </div>
   )
 }