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
  */