Initial commit: DHCP Web Manager

Flask-based web UI for managing ISC DHCP server (dhcpd).

Features:
- subnet / static host binding CRUD via web
- listen interface configuration (INTERFACESv4)
- active lease query + pool utilization stats
- service status, reload/restart, syntax check
- user auth + operation logs
- automatic backup before each config change
- safe change pipeline: write file → dhcpd -t → systemctl restart + is-active verify
- error messages include journalctl tail for diagnosis

templates/subnets.html

@@ -0,0 +1,54 @@
+{% extends "base.html" %}
+{% block title %}子网配置{% endblock %}
+{% block content %}
+<div class="page-head">
+    <h1>子网配置</h1>
+    {% if is_admin %}<a href="{{ url_for('subnet_new') }}" class="btn btn-primary">+ 新增子网</a>{% endif %}
+</div>
+
+{% if subnets %}
+<table class="data-table">
+    <thead>
+        <tr>
+            <th>网段</th>
+            <th>掩码</th>
+            <th>网关</th>
+            <th>DNS</th>
+            <th>地址池</th>
+            <th>租期(默认/最大)</th>
+            <th>authoritative</th>
+            <th>操作</th>
+        </tr>
+    </thead>
+    <tbody>
+    {% for s in subnets %}
+    <tr>
+        <td><code>{{ s.network }}</code></td>
+        <td><code>{{ s.netmask }}</code></td>
+        <td><code>{{ s.routers }}</code></td>
+        <td><code>{{ s.dns }}</code></td>
+        <td><code>{{ s.range_start }}<br>~ {{ s.range_end }}</code></td>
+        <td>{{ s.default_lease }}s / {{ s.max_lease }}s</td>
+        <td>{% if s.authoritative %}<span class="badge badge-ok">yes</span>{% else %}<span class="muted">no</span>{% endif %}</td>
+        <td class="actions">
+            {% if is_admin %}
+            <a href="{{ url_for('subnet_edit', network=s.network, netmask=s.netmask) }}" class="btn btn-sm">编辑</a>
+            <form method="post" action="{{ url_for('subnet_delete', network=s.network, netmask=s.netmask) }}"
+                  onsubmit="return confirm('确认删除子网 {{ s.network }}/{{ s.netmask }}？');" style="display:inline">
+                <button type="submit" class="btn btn-sm btn-danger">删除</button>
+            </form>
+            {% else %}
+            <span class="muted">—</span>
+            {% endif %}
+        </td>
+    </tr>
+    {% endfor %}
+    </tbody>
+</table>
+{% else %}
+<div class="card empty">
+    <p>暂无子网配置</p>
+    {% if is_admin %}<a href="{{ url_for('subnet_new') }}" class="btn btn-primary">+ 新增第一个子网</a>{% endif %}
+</div>
+{% endif %}
+{% endblock %}