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(),
     );
   }
 }

frontend/lib/model/snmp_scanner_status.dart

@@ -2,11 +2,15 @@ class SnmpScannerStatus {
   final bool isRunning;
   final double? runningForSeconds;
   final double? nextRunInSeconds;
+  final int? lastScanDevicesSeen;
+  final double? lastScanSecondsAgo;
 
   const SnmpScannerStatus({
     required this.isRunning,
     this.runningForSeconds,
     this.nextRunInSeconds,
+    this.lastScanDevicesSeen,
+    this.lastScanSecondsAgo,
   });
 
   factory SnmpScannerStatus.fromJson(Map<String, dynamic> json) {
@@ -14,6 +18,8 @@ class SnmpScannerStatus {
       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(),
     );
   }
 }

frontend/lib/widgets/arp_scanner_card.dart

@@ -24,14 +24,20 @@ class ArpScannerCard extends StatelessWidget {
     ArpScannerStatus status,
     double elapsed,
   ) {
-    final successColor =
-        Theme.of(context).extension<AppColorExtension>()!.success;
+    final successColor = Theme.of(
+      context,
+    ).extension<AppColorExtension>()!.success;
     final neutralColor = Theme.of(context).colorScheme.outline;
+    final lastScan = status.lastScanDevicesSeen != null
+        ? '${status.lastScanDevicesSeen} devices on last scan'
+        : null;
     if (status.isRunning) {
-      final sub = status.runningForSeconds != null
-          ? ['Running for ${formatSeconds(status.runningForSeconds! + elapsed)}']
-          : <String>[];
-      return (color: successColor, label: 'Running', sublabels: sub);
+      final sublabels = <String>[
+        if (status.runningForSeconds != null)
+          'Running for ${formatSeconds(status.runningForSeconds! + elapsed)}',
+        ?lastScan,
+      ];
+      return (color: successColor, label: 'Running', sublabels: sublabels);
     }
     if (status.nextRunInSeconds != null) {
       final remaining = (status.nextRunInSeconds! - elapsed).clamp(
@@ -41,7 +47,7 @@ class ArpScannerCard extends StatelessWidget {
       return (
         color: neutralColor,
         label: 'Waiting for next run',
-        sublabels: ['Next run in ${formatSeconds(remaining)}'],
+        sublabels: ['Next run in ${formatSeconds(remaining)}', ?lastScan],
       );
     }
     return (color: neutralColor, label: 'Not started', sublabels: []);

frontend/lib/widgets/snmp_scanner_card.dart

@@ -28,11 +28,16 @@ class SnmpScannerCard extends StatelessWidget {
       context,
     ).extension<AppColorExtension>()!.success;
     final neutralColor = Theme.of(context).colorScheme.outline;
+    final lastScan = status.lastScanDevicesSeen != null
+        ? '${status.lastScanDevicesSeen} devices on last scan'
+        : null;
     if (status.isRunning) {
-      final sub = status.runningForSeconds != null
-          ? ['Running for ${formatSeconds(status.runningForSeconds! + elapsed)}']
-          : <String>[];
-      return (color: successColor, label: 'Running', sublabels: sub);
+      final sublabels = <String>[
+        if (status.runningForSeconds != null)
+          'Running for ${formatSeconds(status.runningForSeconds! + elapsed)}',
+        ?lastScan,
+      ];
+      return (color: successColor, label: 'Running', sublabels: sublabels);
     }
     if (status.nextRunInSeconds != null) {
       final remaining = (status.nextRunInSeconds! - elapsed).clamp(
@@ -42,7 +47,7 @@ class SnmpScannerCard extends StatelessWidget {
       return (
         color: neutralColor,
         label: 'Waiting for next run',
-        sublabels: ['Next run in ${formatSeconds(remaining)}'],
+        sublabels: ['Next run in ${formatSeconds(remaining)}', ?lastScan],
       );
     }
     return (color: neutralColor, label: 'Not started', sublabels: []);