fix(ipam): 修复IP管理页面网段筛选不生效及下拉框过窄

- Ips.vue: el-select 添加 width: 280px，修复网段下拉框过窄
- Ips.vue: query 参数 networkId → network_id（与 FastAPI snake_case 一致）
- Scan.vue: el-select 用 network.id 代替整个 network 对象作为 value，修复切换网段不生效
- enhanced_scan_service.py: 替换失效的 scapy ARP 扫描为 arp-scan 命令；扫描结果自动创建缺失 IP 记录

backend/.env.example

@@ -0,0 +1,19 @@
+# 数据库配置
+MYSQL_ROOT_PASSWORD=ipam2024
+MYSQL_DATABASE=ipam
+MYSQL_USER=ipam
+MYSQL_PASSWORD=ipam2024
+
+# 应用配置
+DATABASE_URL=mysql+pymysql://ipam:***@localhost:3306/ipam
+REDIS_URL=redis://localhost:6379/0
+CELERY_BROKER_URL=redis://localhost:6379/1
+CELERY_RESULT_BACKEND=redis://localhost:6379/2
+
+# JWT配置
+SECRET_KEY=your-super-secret-key-here-change-in-production
+
+# 扫描配置
+PING_TIMEOUT=2
+PING_RETRIES=2
+SCAN_CONCURRENCY=50