Add network scan page (arp-scan)

New /scan page lists all active devices on the subnet (ARP scan):
- 静态绑定: matches a host declaration
- 动态分配: matches a dhcpd.leases entry
- 未知 / 外部: nothing matches (manually configured or from other DHCP server)

Click '转为静态绑定' to convert a discovered device to a static host binding.

Also includes:
- arp-scan wrapper in dhcp_ops.py (get_local_network, arp_scan, classify_devices)
- /scan route with optional auto-refresh every 30s
- README updated with arp-scan install step + new feature bullet

app.py

@@ -459,6 +459,34 @@ def lease_list():
     return render_template("leases.html", leases=leases, q=q)
 
 
+# ---------- 网络扫描 (arp-scan) ----------
+
+@app.route("/scan", methods=["GET", "POST"])
+@login_required
+def scan():
+    """网络扫描 - 用 arp-scan 列出网段内所有活跃设备"""
+    ifaces = dhcp_ops.get_interfaces()
+    iface = request.values.get("iface") or (ifaces[0] if ifaces else "enp0s31f6-ovs")
+    network = request.values.get("network") or dhcp_ops.get_local_network(iface)
+
+    devices = []
+    scan_msg = ""
+    did_scan = False
+
+    if request.method == "POST" or request.args.get("auto") == "1":
+        did_scan = True
+        ok, scan_msg, devices = dhcp_ops.arp_scan(iface, network)
+        if ok:
+            # 交叉 dhcpd.leases 和 host 静态绑定
+            hosts = get_hosts()
+            leases = dhcp_ops.parse_leases()
+            devices = dhcp_ops.classify_devices(devices, hosts, leases)
+
+    return render_template("scan.html",
+                           ifaces=ifaces, iface=iface, network=network,
+                           devices=devices, scan_msg=scan_msg, did_scan=did_scan)
+
+
 # ---------- config (interfaces + 原始 conf) ----------
 
 @app.route("/config", methods=["GET", "POST"])