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

templates/base.html

@@ -16,6 +16,7 @@
             <a href="{{ url_for('subnet_list') }}" class="{% if request.endpoint in ['subnet_list','subnet_new','subnet_edit'] %}active{% endif %}">子网</a>
             <a href="{{ url_for('host_list') }}" class="{% if request.endpoint in ['host_list','host_new','host_edit'] %}active{% endif %}">静态绑定</a>
             <a href="{{ url_for('lease_list') }}" class="{% if request.endpoint=='lease_list' %}active{% endif %}">租约</a>
+            <a href="{{ url_for('scan') }}" class="{% if request.endpoint=='scan' %}active{% endif %}">扫描</a>
             <a href="{{ url_for('config_view') }}" class="{% if request.endpoint=='config_view' %}active{% endif %}">服务配置</a>
             {% if is_admin %}
             <a href="{{ url_for('log_list') }}" class="{% if request.endpoint=='log_list' %}active{% endif %}">日志</a>

templates/scan.html

@@ -0,0 +1,110 @@
+{% extends "base.html" %}
+{% block title %}网络扫描{% endblock %}
+{% block content %}
+<div class="page-head">
+    <h1>网络扫描</h1>
+    <span class="muted small">用 arp-scan 列出网段内活跃设备</span>
+</div>
+
+<div class="card">
+    <form method="post" class="form-inline">
+        <label>接口
+            <select name="iface">
+                {% for i in ifaces %}
+                <option value="{{ i }}" {% if i==iface %}selected{% endif %}>{{ i }}</option>
+                {% endfor %}
+                {% if not ifaces %}
+                <option value="{{ iface }}">{{ iface }}</option>
+                {% endif %}
+            </select>
+        </label>
+        <label>网段
+            <input type="text" name="network" value="{{ network or '' }}" placeholder="10.168.1.0/24">
+        </label>
+        <button type="submit" class="btn btn-primary">扫描</button>
+        <label class="checkbox-row" style="margin-left:12px">
+            <input type="checkbox" id="autoRefresh" checked>
+            自动刷新（30 秒）
+        </label>
+    </form>
+    {% if did_scan %}
+    <p class="muted small" style="margin-top:8px">{{ scan_msg }}</p>
+    {% endif %}
+</div>
+
+{% if did_scan %}
+<div class="card">
+    <h2>扫描结果（{{ devices|length }} 个设备）</h2>
+    {% if devices %}
+    <table class="data-table">
+        <thead>
+            <tr>
+                <th>IP</th>
+                <th>MAC</th>
+                <th>来源</th>
+                <th>操作</th>
+            </tr>
+        </thead>
+        <tbody>
+        {% for d in devices %}
+        <tr>
+            <td><code>{{ d.ip }}</code></td>
+            <td><code>{{ d.mac }}</code></td>
+            <td>
+                {% if d.source == 'static' %}
+                    <span class="badge badge-ok">静态绑定</span>
+                {% elif d.source == 'dynamic' %}
+                    <span class="badge badge-warn">动态分配</span>
+                {% else %}
+                    <span class="badge">未知 / 外部</span>
+                {% endif %}
+            </td>
+            <td class="actions">
+                {% if d.source != 'static' and is_admin %}
+                <form method="post" action="{{ url_for('host_new') }}" style="display:inline">
+                    <input type="hidden" name="mac" value="{{ d.mac }}">
+                    <input type="hidden" name="ip" value="{{ d.ip }}">
+                    <input type="hidden" name="name" value="dev-{{ d.ip|replace('.', '-') }}">
+                    <button type="submit" class="btn btn-sm">转为静态绑定</button>
+                </form>
+                {% endif %}
+            </td>
+        </tr>
+        {% endfor %}
+        </tbody>
+    </table>
+    <p class="muted small">
+        说明：「静态绑定」= 在 Web 静态绑定（host 声明）里；
+        「动态分配」= 在 dhcpd.leases 里被分配过；
+        「未知/外部」= 不在上述两类（可能手动配的或别的 DHCP 服务器给的）。
+    </p>
+    {% else %}
+    <p class="muted">未发现活跃设备（网段内无应答）</p>
+    {% endif %}
+</div>
+{% else %}
+<div class="card empty">
+    <p>点击「扫描」开始 ARP 扫描。</p>
+    <p class="muted small">提示：扫描时会向网段每个 IP 发 ARP 请求，3-5 秒完成。</p>
+</div>
+{% endif %}
+
+<script>
+let timer = null;
+function setupAutoRefresh() {
+    const cb = document.getElementById('autoRefresh');
+    if (!cb) return;
+    if (timer) clearTimeout(timer);
+    if (!cb.checked) return;
+    // 只在已经扫过一次的情况下自动刷新
+    if (document.querySelector('table.data-table')) {
+        timer = setTimeout(() => {
+            const f = document.querySelector('form.form-inline');
+            if (f) f.submit();
+        }, 30000);
+    }
+}
+document.addEventListener('DOMContentLoaded', setupAutoRefresh);
+document.getElementById('autoRefresh')?.addEventListener('change', setupAutoRefresh);
+</script>
+{% endblock %}