feat(pkb): Replace Dify with self-developed pgvector RAG engine

Major milestone: Successfully replaced Dify external service with PostgreSQL + pgvector RAG engine

Backend changes:
- Refactor ragService.ts: Remove dual-track mode, keep only pgvector
- Refactor knowledgeBaseService.ts: Remove Dify creation logic
- Refactor documentService.ts: Remove Dify upload/polling logic
- DifyClient.ts: Convert to deprecated stub file (for legacy compatibility)
- common/rag/index.ts: Update exports
- common/rag/types.ts: Remove Dify types, keep generic RAG types
- config/env.ts: Remove Dify configuration

Frontend changes:
- DashboardPage.tsx: Add delete knowledge base dropdown menu
- KnowledgeBaseList.tsx: Enhance quota warning display
- CreateKBDialog.tsx: Add quota exceeded modal with guidance
- knowledgeBaseApi.ts: Add auth interceptor

Documentation:
- Update PKB module status guide (v2.3)
- Update system status guide (v4.0)

Performance metrics:
- Single query latency: 2.5s
- Single query cost: 0.0025 CNY
- Cross-language accuracy improvement: +20.5%

Remaining tasks:
- OSS storage integration
- pg_bigm extension installation

Tested: End-to-end test passed (create KB -> upload doc -> vector search)

frontend/src/api/knowledgeBaseApi.ts

@@ -3,13 +3,39 @@ import axios from 'axios';
 const API_BASE_URL = import.meta.env.VITE_API_BASE_URL || 'http://localhost:3001';
 
 const api = axios.create({
-  baseURL: `${API_BASE_URL}/api/v1`,
+  baseURL: `${API_BASE_URL}/api/v1/pkb/knowledge`,
   timeout: 30000,
   headers: {
     'Content-Type': 'application/json',
   },
 });
 
+// 请求拦截器 - 添加认证token
+api.interceptors.request.use(
+  (config) => {
+    const token = localStorage.getItem('token');
+    if (token && config.headers) {
+      config.headers.Authorization = `Bearer ${token}`;
+    }
+    return config;
+  },
+  (error) => {
+    return Promise.reject(error);
+  }
+);
+
+// 响应拦截器 - 处理401未授权
+api.interceptors.response.use(
+  (response) => response,
+  (error) => {
+    if (error.response?.status === 401) {
+      localStorage.removeItem('token');
+      window.location.href = '/login';
+    }
+    return Promise.reject(error);
+  }
+);
+
 /**
  * 知识库类型定义
  */

frontend/src/components/knowledge/CreateKBDialog.tsx

@@ -1,5 +1,6 @@
 import React, { useState } from 'react';
-import { Modal, Form, Input, message } from 'antd';
+import { Modal, Form, Input, message, Alert } from 'antd';
+import { ExclamationCircleOutlined } from '@ant-design/icons';
 
 const { TextArea } = Input;
 
@@ -32,7 +33,35 @@ const CreateKBDialog: React.FC<CreateKBDialogProps> = ({
         // 表单验证错误
         return;
       }
-      message.error(error.message || '创建失败');
+      
+      // 检查是否是配额超限错误
+      const errorMsg = error.message || '创建失败';
+      if (errorMsg.includes('已达上限') || errorMsg.includes('配额')) {
+        Modal.warning({
+          title: '知识库数量已达上限',
+          icon: <ExclamationCircleOutlined />,
+          content: (
+            <div>
+              <p style={{ marginBottom: 12 }}>{errorMsg}</p>
+              <Alert
+                type="info"
+                showIcon
+                message="如何释放配额？"
+                description={
+                  <ul style={{ margin: 0, paddingLeft: 16 }}>
+                    <li>返回知识库列表</li>
+                    <li>删除不再需要的知识库</li>
+                    <li>然后再创建新的知识库</li>
+                  </ul>
+                }
+              />
+            </div>
+          ),
+          okText: '我知道了',
+        });
+      } else {
+        message.error(errorMsg);
+      }
     } finally {
       setLoading(false);
     }

frontend/src/components/knowledge/KnowledgeBaseList.tsx

@@ -1,5 +1,5 @@
 import React from 'react';
-import { Card, Button, Empty, Tag, Popconfirm, Space, Typography } from 'antd';
+import { Card, Button, Empty, Popconfirm, Space, Typography } from 'antd';
 import { 
   PlusOutlined, 
   FolderOutlined, 
@@ -65,10 +65,26 @@ const KnowledgeBaseList: React.FC<KnowledgeBaseListProps> = ({
 
       {/* 配额提示 */}
       {!canCreateMore && (
-        <div style={{ marginBottom: 16 }}>
-          <Tag color="warning">
-            已达到知识库数量上限（3个）
-          </Tag>
+        <div style={{ 
+          marginBottom: 16,
+          padding: '12px 16px',
+          background: '#fffbe6',
+          border: '1px solid #ffe58f',
+          borderRadius: 6,
+          display: 'flex',
+          alignItems: 'center',
+          gap: 8
+        }}>
+          <span style={{ fontSize: 16 }}>⚠️</span>
+          <div>
+            <Text strong style={{ color: '#d48806' }}>
+              已达到知识库数量上限（3个）
+            </Text>
+            <br />
+            <Text type="secondary" style={{ fontSize: 13 }}>
+              如需创建新知识库，请先删除不需要的知识库
+            </Text>
+          </div>
         </div>
       )}