feat: 密码强度多级校验 — low/medium/high/very_high

- site_settings 新增 password.strength 配置项
- 四级：low(len>=8) / medium(两类) / high(三类+无连续) / very_high(四类+无连续)
- 连续字符检查：重复/升序/降序/键盘水平/键盘垂直序列
- 注册和改密接口统一读取配置
- 前端 register.js 动态校验 + 管理后台安全设置区

templates/admin/html/site-settings/index.html

@@ -165,6 +165,28 @@
             </div>
         </div>
 
+        <!-- 安全设置 -->
+        <div class="settings-section">
+            <div class="section-title">安全设置</div>
+            <div class="section-body">
+                <div class="setting-item">
+                    <div class="setting-info">
+                        <span class="setting-label">密码强度</span>
+                        <span class="setting-desc">注册和修改密码时的最低强度要求</span>
+                    </div>
+                    <div class="setting-input-wrap">
+                        <select id="setting-password-strength" class="setting-select" data-key="password.strength">
+                            <option value="low">低 — 仅要求 8 位</option>
+                            <option value="medium" selected>中 — 字母+数字/符号（两类）</option>
+                            <option value="high">高 — 大小写+数字+符号（至少三类）+ 禁止连续字符</option>
+                            <option value="very_high">极高 — 四类全有 + 禁止连续字符</option>
+                        </select>
+                        <button class="btn btn-sm btn-primary btn-save-select" data-key="password.strength" data-input="setting-password-strength">保存</button>
+                    </div>
+                </div>
+            </div>
+        </div>
+
         <!-- 注册设置 -->
         <div class="settings-section">
             <div class="section-title">注册设置</div>