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/app/api/v1/__init__.py

@@ -0,0 +1,14 @@
+from fastapi import APIRouter
+from app.api.v1 import networks, ips, scan, enhanced_scan, async_scan, snmp, alerts, auth, audit
+
+api_router = APIRouter(prefix="/api/v1")
+
+api_router.include_router(networks.router)
+api_router.include_router(ips.router)
+api_router.include_router(scan.router)
+api_router.include_router(enhanced_scan.router)
+api_router.include_router(async_scan.router)
+api_router.include_router(snmp.router)
+api_router.include_router(alerts.router)
+api_router.include_router(auth.router)
+api_router.include_router(audit.router)