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

dhcp_ops.py

@@ -515,4 +515,91 @@ def get_service_status() -> Dict:
         info["status_text"] = "\n".join(info["status_text"][:5])
     except (subprocess.TimeoutExpired, FileNotFoundError):
         pass
-    return info
+    return info
+
+
+# ---------------------------------------------------------------------------
+# 网络扫描（arp-scan）
+# ---------------------------------------------------------------------------
+
+@dataclass
+class ArpDevice:
+    ip: str
+    mac: str
+    vendor: str = ""
+    source: str = ""   # 标记：static / dynamic / unknown
+
+
+def get_local_network(iface: str = "enp0s31f6-ovs") -> Optional[str]:
+    """从指定接口的 IPv4 地址 + 掩码计算出 /24 网络号"""
+    try:
+        out = subprocess.run(
+            ["ip", "-4", "-o", "addr", "show", "dev", iface],
+            capture_output=True, text=True, timeout=5,
+        ).stdout
+        # 形如: "4: enp0s31f6-ovs    inet 10.168.1.209/24 brd 10.168.1.255 scope global enp0s31f6-ovs\"
+        import re as _re
+        m = _re.search(r"inet\s+(\d+\.\d+\.\d+\.\d+)/(\d+)", out)
+        if not m:
+            return None
+        ip, prefix = m.group(1), int(m.group(2))
+        if prefix != 24:
+            # 仅支持 /24 扫描；其他掩码返回 None 让 UI 提示用户
+            return None
+        return ".".join(ip.split(".")[:3]) + ".0/24"
+    except Exception:
+        return None
+
+
+def arp_scan(iface: str = "enp0s31f6-ovs", network: Optional[str] = None,
+             timeout: int = 10) -> Tuple[bool, str, List[ArpDevice]]:
+    """
+    扫描指定接口/网段，返回 (ok, msg, devices)。
+    - 用 arp-scan（系统二进制）
+    - 如果传 network=None，自动从 iface 推算
+    """
+    if network is None:
+        network = get_local_network(iface) or "10.168.1.0/24"
+
+    cmd = ["arp-scan", f"--interface={iface}", "--numeric", "--quiet", "--retry=1",
+           "--timeout=" + str(timeout * 1000), network]
+    try:
+        r = subprocess.run(cmd, capture_output=True, text=True, timeout=timeout + 5)
+        if r.returncode not in (0, 1):  # arp-scan: 0=全部收到, 1=部分收到
+            return False, (r.stderr or r.stdout).strip(), []
+    except FileNotFoundError:
+        return False, "未安装 arp-scan（apt-get install arp-scan）", []
+    except subprocess.TimeoutExpired:
+        return False, "arp-scan 超时", []
+
+    devices: List[ArpDevice] = []
+    for line in r.stdout.splitlines():
+        line = line.strip()
+        if not line or line.startswith("Interface:") or "arp-scan" in line or "packets" in line:
+            continue
+        parts = line.split()
+        if len(parts) >= 2 and "." in parts[0] and ":" in parts[1]:
+            devices.append(ArpDevice(ip=parts[0], mac=parts[1].lower()))
+    msg = f"扫描完成，发现 {len(devices)} 个设备"
+    return True, msg, devices
+
+
+def classify_devices(devices: List[ArpDevice], hosts, leases) -> List[ArpDevice]:
+    """
+    给设备打标记：
+    - static: 在静态绑定（host 声明）里
+    - dynamic: 在 dhcpd.leases 里
+    - unknown: 都不在（手动配的或别处 DHCP 给的）
+    """
+    static_macs = {h.mac.lower() for h in hosts}
+    static_ips = {h.ip for h in hosts}
+    lease_macs = {l.mac.lower() for l in leases if l.mac}
+    lease_ips = {l.ip for l in leases}
+    for d in devices:
+        if d.mac in static_macs or d.ip in static_ips:
+            d.source = "static"
+        elif d.mac in lease_macs or d.ip in lease_ips:
+            d.source = "dynamic"
+        else:
+            d.source = "unknown"
+    return devices