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/package.json

@@ -12,7 +12,8 @@
     "lucide-react": "^0.468.0",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
-    "react-router-dom": "^6.28.0"
+    "react-router-dom": "^6.28.0",
+    "recharts": "^3.9.2"
   },
   "devDependencies": {
     "@types/react": "^18.3.1",