Report devices seen on last scan in ARP/SNMP status

The ARP and SNMP scanner status endpoints and front-end cards now expose
the number of devices found by the most recent successful scan, following
the existing mDNS device-count pattern. The count persists across the
running/waiting transitions, and for SNMP a failed poll keeps the last
good count rather than overwriting it.

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

frontend/lib/model/arp_scanner_status.dart

@@ -2,11 +2,15 @@ class ArpScannerStatus {
   final bool isRunning;
   final double? runningForSeconds;
   final double? nextRunInSeconds;
+  final int? lastScanDevicesSeen;
+  final double? lastScanSecondsAgo;
 
   const ArpScannerStatus({
     required this.isRunning,
     this.runningForSeconds,
     this.nextRunInSeconds,
+    this.lastScanDevicesSeen,
+    this.lastScanSecondsAgo,
   });
 
   factory ArpScannerStatus.fromJson(Map<String, dynamic> json) {
@@ -14,6 +18,8 @@ class ArpScannerStatus {
       isRunning: json['is_running'] as bool,
       runningForSeconds: (json['running_for_seconds'] as num?)?.toDouble(),
       nextRunInSeconds: (json['next_run_in_seconds'] as num?)?.toDouble(),
+      lastScanDevicesSeen: (json['last_scan_devices_seen'] as num?)?.toInt(),
+      lastScanSecondsAgo: (json['last_scan_seconds_ago'] as num?)?.toDouble(),
     );
   }
 }