feat: 钉钉机器人支持加签安全设置

- 通知渠道增加 sign_secret 字段存储加签密钥
- 发送钉钉消息时自动计算签名
- 前端增加加签密钥输入框（仅钉钉机器人显示）

frontend/src/views/notification-channels/index.vue

@@ -25,6 +25,7 @@ const form = reactive({
   channel_name: '',
   channel_type: 'dingtalk_bot',
   webhook_url: '',
+  sign_secret: '',
   description: ''
 })
 
@@ -78,6 +79,7 @@ function handleCreate() {
     channel_name: '',
     channel_type: 'dingtalk_bot',
     webhook_url: '',
+    sign_secret: '',
     description: ''
   })
   dialogVisible.value = true
@@ -91,6 +93,7 @@ function handleEdit(row) {
     channel_name: row.channel_name,
     channel_type: row.channel_type,
     webhook_url: row.webhook_url,
+    sign_secret: row.sign_secret || '',
     description: row.description || ''
   })
   dialogVisible.value = true
@@ -258,6 +261,12 @@ onMounted(() => {
             </template>
           </div>
         </el-form-item>
+        <el-form-item v-if="form.channel_type === 'dingtalk_bot'" label="加签密钥">
+          <el-input v-model="form.sign_secret" placeholder="SEC开头的密钥（可选）" />
+          <div class="form-tip">
+            如果创建机器人时选择了「加签」安全设置，请填写密钥（以 SEC 开头）
+          </div>
+        </el-form-item>
         <el-form-item label="描述">
           <el-input v-model="form.description" type="textarea" :rows="2" placeholder="渠道描述（可选）" />
         </el-form-item>