refactor: Webhook 移入系统设置「消息通知」Tab，取消独立菜单

- WebhookConfig 页面转为可嵌入卡片组件 WebhookManager
  （标准设置卡样式，逻辑不变），渲染于飞书通知卡下方
- 移除侧边栏「Webhook」菜单项、/webhooks 路由与页面文件
  （旧地址自动回退流水线页）
- 使用指南/接口文档同步更新 Webhook 配置入口描述

frontend/src/components/Sidebar.tsx

@@ -10,7 +10,6 @@ import {
   Rocket,
   Zap,
   History,
-  Webhook,
   CloudSync,
   Puzzle,
   Settings,
@@ -24,7 +23,7 @@ import { useQuery } from '@tanstack/react-query';
 import { apiClient } from '@/lib/api';
 import { useTheme } from '@/providers/theme';
 
-type ModuleType = 'pipelines' | 'gitops' | 'batch-release' | 'release-records' | 'environments' | 'credentials' | 'build-nodes' | 'tools' | 'jenkins-import' | 'webhooks' | 'plugins' | 'users' | 'docs' | 'settings';
+type ModuleType = 'pipelines' | 'gitops' | 'batch-release' | 'release-records' | 'environments' | 'credentials' | 'build-nodes' | 'tools' | 'jenkins-import' | 'plugins' | 'users' | 'docs' | 'settings';
 
 interface SidebarProps {
   activeModule: ModuleType;
@@ -56,7 +55,6 @@ const menuGroups: { label: string; items: { id: ModuleType; label: string; icon:
     label: '集成',
     items: [
       { id: 'jenkins-import', label: 'Jenkins导入', icon: Download, badge: null },
-      { id: 'webhooks', label: 'Webhook', icon: Webhook, badge: null },
       { id: 'tools', label: '工具配置', icon: Wrench, badge: null },
     ],
   },