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/lib/model/device_event_type.dart

@@ -1,7 +1,9 @@
 /// Type of a recorded device event, mirroring the backend's `DeviceEventType`.
 enum DeviceEventType {
   newDevice('NewDevice'),
-  deviceSeen('DeviceSeen');
+  deviceSeen('DeviceSeen'),
+  deviceChanged('DeviceChanged'),
+  deviceBackOnline('DeviceBackOnline');
 
   const DeviceEventType(this.wireValue);