Add DeviceChanged and DeviceBackOnline event types

Record what happened on each known-device sighting instead of only
"seen": a baseline DeviceSeen (history heartbeat, no notification) plus
DeviceChanged and DeviceBackOnline events, each deduplicated
independently so a recent routine sighting no longer suppresses a
genuine change or return notification. The frontend chart now trusts the
event type for its marker and tooltip rather than comparing each event's
snapshot against the device's current state.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/test/widget/device_event_history_test.dart

@@ -0,0 +1,59 @@
+import 'package:fl_chart/fl_chart.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frontend/devices/device_event_history.dart';
+import 'package:frontend/model/device.dart';
+import 'package:http_mock_adapter/http_mock_adapter.dart';
+
+import '../helpers/backend_test_harness.dart';
+import '../helpers/fixtures.dart';
+import '../helpers/pump_app.dart';
+
+void main() {
+  late DioAdapter adapter;
+  const mac = 'aa:bb:cc:dd:ee:ff';
+
+  setUp(() async {
+    adapter = await setUpBackendForTest();
+  });
+
+  Device device() => Device.fromJson(deviceJson(macAddress: mac));
+
+  testWidgets('renders the chart for events of every type', (tester) async {
+    adapter.onGet(
+      '/devices/$mac/events',
+      (server) => server.reply(200, [
+        deviceEventJson(id: 1, eventType: 'NewDevice'),
+        deviceEventJson(id: 2, eventType: 'DeviceSeen'),
+        deviceEventJson(id: 3, eventType: 'DeviceChanged'),
+        deviceEventJson(id: 4, eventType: 'DeviceBackOnline'),
+      ]),
+      // The widget always sends a (dynamic, now-relative) created_from filter.
+      queryParameters: {'created_from': Matchers.any},
+    );
+
+    await pumpScreen(tester, DeviceEventHistory(device: device()));
+    await pumpUntilFound(tester, find.byType(ScatterChart));
+
+    expect(find.byType(ScatterChart), findsOneWidget);
+    expect(find.text('No events in this time range'), findsNothing);
+
+    await tearDownTree(tester);
+  });
+
+  testWidgets('shows an empty-state message when there are no events', (
+    tester,
+  ) async {
+    adapter.onGet(
+      '/devices/$mac/events',
+      (server) => server.reply(200, <dynamic>[]),
+      queryParameters: {'created_from': Matchers.any},
+    );
+
+    await pumpScreen(tester, DeviceEventHistory(device: device()));
+    await pumpUntilFound(tester, find.text('No events in this time range'));
+
+    expect(find.byType(ScatterChart), findsNothing);
+
+    await tearDownTree(tester);
+  });
+}