Show scanner source in device event tooltip

Surface the originating scanner ("ARP" or "mDNS") in the event timeline
tooltip so users can tell which scanner reported each sighting.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/lib/devices/device_event_history.dart

@@ -257,9 +257,10 @@ class _EventChart extends StatelessWidget {
               final event = events[idx];
               final dt = event.createdOn.toLocal();
               final dateStr = DateFormat('MMM d, yyyy HH:mm').format(dt);
-              final typeLabel = event.eventType == 'NewDevice'
+              final baseLabel = event.eventType == 'NewDevice'
                   ? 'First seen'
                   : 'Device seen';
+              final typeLabel = '$baseLabel (${event.scannerLabel})';
 
               final diffs = <TextSpan>[];
               if (event.ipv4Address != device.ipv4Address) {

frontend/lib/model/device_event.dart

@@ -5,6 +5,7 @@ class DeviceEvent {
   final String eventType;
   final String ipv4Address;
   final String vendor;
+  final String scanner;
 
   const DeviceEvent({
     required this.id,
@@ -13,6 +14,7 @@ class DeviceEvent {
     required this.eventType,
     required this.ipv4Address,
     required this.vendor,
+    required this.scanner,
   });
 
   factory DeviceEvent.fromJson(Map<String, dynamic> json) {
@@ -23,6 +25,13 @@ class DeviceEvent {
       eventType: json['event_type'] as String,
       ipv4Address: json['ipv4_address'] as String,
       vendor: json['vendor'] as String,
+      scanner: json['scanner'] as String,
     );
   }
+
+  String get scannerLabel => switch (scanner) {
+    'Arp' => 'ARP',
+    'Mdns' => 'mDNS',
+    _ => scanner,
+  };
 }