fix(snmp): ARP表缺少发现时间字段，在get_arp_table返回值中添加last_seen和discovered_at

backend/app/services/snmp_service.py

@@ -221,18 +221,20 @@ class SNMPService:
                                 arp_data[ip_address]['if_index'] = if_index
 
                 # 转换为列表
+                now = datetime.utcnow()
                 for ip, data in arp_data.items():
                     if 'mac_address' in data:
                         entry = {
                             'ip_address': ip,
                             'mac_address': data.get('mac_address'),
                             'interface': str(data.get('if_index', '')),
-                            'device_id': device.id
+                            'device_id': device.id,
+                            'last_seen': now.isoformat(),
+                            'discovered_at': now.isoformat()
                         }
                         arp_entries.append(entry)
 
             # 保存到数据库
-            now = datetime.utcnow()
             for entry_data in arp_entries:
                 # 查找是否已存在
                 existing = db.query(ARPEntry).filter(