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/types.ts

@@ -215,6 +215,21 @@ export interface DevicePhoto {
   uploaded_at: string
 }
 
+// One row of GET /api/devices/:id/history — sparse by design (see recordReading()
+// in backend/src/db.js): whichever fields the event that produced this row actually
+// knew about are set, the rest are null. The history chart treats each column
+// independently rather than assuming a full snapshot on every row.
+export interface DeviceReading {
+  room_temp: string | null
+  target_temp: string | null
+  mode: string | null
+  power_state: 'on' | 'off' | null
+  fan_speed: string | null
+  valve_position: number | null
+  heating_active: boolean | null
+  recorded_at: string
+}
+
 export interface ActivityEntry {
   id: number
   zone_id: number | null