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-v2/src/modules/pkb/pages/DashboardPage.tsx

@@ -3,7 +3,7 @@
  * 严格遵循：知识库仪表盘V5.html
  */
 
-import React, { useState, useEffect } from 'react';
+import React, { useState, useEffect, useRef } from 'react';
 import { useNavigate } from 'react-router-dom';
 import { useKnowledgeBaseStore } from '../stores/useKnowledgeBaseStore';
 import DocumentUpload from '../components/DocumentUpload';
@@ -11,9 +11,9 @@ import {
   Plus, BookOpen, Microscope, Stethoscope, Pill, 
   GraduationCap, Wrench, MessageSquare, FileText, 
   Loader2, MoreHorizontal, X, CheckCircle2, 
-  ChevronRight, Upload, Sparkles, Trash2, ArrowRight
+  ChevronRight, Upload, Sparkles, Trash2, ArrowRight, Edit3
 } from 'lucide-react';
-import { message } from 'antd';
+import { message, Modal } from 'antd';
 import type { KBType } from '../types/workspace';
 
 // 6种知识库类型配置（严格遵循V5设计）
@@ -76,7 +76,7 @@ const KB_TYPES = [
 
 const DashboardPage: React.FC = () => {
   const navigate = useNavigate();
-  const { knowledgeBases, fetchKnowledgeBases, createKnowledgeBase } = useKnowledgeBaseStore();
+  const { knowledgeBases, fetchKnowledgeBases, createKnowledgeBase, deleteKnowledgeBase } = useKnowledgeBaseStore();
   
   // Modal状态
   const [isModalOpen, setIsModalOpen] = useState(false);
@@ -89,10 +89,52 @@ const DashboardPage: React.FC = () => {
   // 新增：创建知识库后保存ID，用于Step3上传文档
   const [createdKbId, setCreatedKbId] = useState<string | null>(null);
   const [uploadedCount, setUploadedCount] = useState(0);
+  
+  // 下拉菜单状态
+  const [openMenuId, setOpenMenuId] = useState<string | null>(null);
+  const menuRef = useRef<HTMLDivElement>(null);
 
   useEffect(() => {
     fetchKnowledgeBases();
   }, []);
+  
+  // 点击外部关闭菜单
+  useEffect(() => {
+    const handleClickOutside = (event: MouseEvent) => {
+      if (menuRef.current && !menuRef.current.contains(event.target as Node)) {
+        setOpenMenuId(null);
+      }
+    };
+    document.addEventListener('mousedown', handleClickOutside);
+    return () => document.removeEventListener('mousedown', handleClickOutside);
+  }, []);
+  
+  // 删除知识库
+  const handleDeleteKb = async (kbId: string, kbName: string) => {
+    Modal.confirm({
+      title: '确认删除知识库？',
+      content: (
+        <div>
+          <p>您即将删除知识库 <strong>"{kbName}"</strong></p>
+          <p className="text-red-500 text-sm mt-2">
+            ⚠️ 此操作将同时删除该知识库中的所有文档，且不可恢复！
+          </p>
+        </div>
+      ),
+      okText: '确认删除',
+      okButtonProps: { danger: true },
+      cancelText: '取消',
+      onOk: async () => {
+        try {
+          await deleteKnowledgeBase(kbId);
+          message.success('知识库删除成功');
+          setOpenMenuId(null);
+        } catch (error: any) {
+          message.error(error.message || '删除失败');
+        }
+      },
+    });
+  };
 
   const getKbTypeConfig = (id: KBType) => KB_TYPES.find(t => t.id === id) || KB_TYPES[0];
 
@@ -198,9 +240,45 @@ const DashboardPage: React.FC = () => {
                     <div className={`p-2.5 rounded-lg ${style.bg} ${style.color}`}>
                       <TypeIcon className="w-6 h-6" />
                     </div>
-                    <button className="text-gray-300 hover:text-gray-600 p-1 rounded hover:bg-gray-100">
-                      <MoreHorizontal className="w-5 h-5" />
-                    </button>
+                    <div className="relative" ref={openMenuId === kb.id ? menuRef : null}>
+                      <button 
+                        onClick={(e) => {
+                          e.stopPropagation();
+                          setOpenMenuId(openMenuId === kb.id ? null : kb.id);
+                        }}
+                        className="text-gray-300 hover:text-gray-600 p-1 rounded hover:bg-gray-100"
+                      >
+                        <MoreHorizontal className="w-5 h-5" />
+                      </button>
+                      
+                      {/* 下拉菜单 */}
+                      {openMenuId === kb.id && (
+                        <div className="absolute right-0 top-8 bg-white rounded-lg shadow-xl border border-gray-200 py-1 z-50 min-w-[140px] animate-in fade-in duration-150">
+                          <button
+                            onClick={(e) => {
+                              e.stopPropagation();
+                              setOpenMenuId(null);
+                              navigate(`/knowledge-base/workspace/${kb.id}`);
+                            }}
+                            className="w-full px-4 py-2 text-left text-sm text-gray-700 hover:bg-gray-50 flex items-center gap-2"
+                          >
+                            <Edit3 className="w-4 h-4" />
+                            编辑知识库
+                          </button>
+                          <div className="border-t border-gray-100 my-1"></div>
+                          <button
+                            onClick={(e) => {
+                              e.stopPropagation();
+                              handleDeleteKb(kb.id, kb.name);
+                            }}
+                            className="w-full px-4 py-2 text-left text-sm text-red-600 hover:bg-red-50 flex items-center gap-2"
+                          >
+                            <Trash2 className="w-4 h-4" />
+                            删除知识库
+                          </button>
+                        </div>
+                      )}
+                    </div>
                   </div>
 
                   <h3 className="font-bold text-lg text-slate-800 mb-2 line-clamp-1 group-hover:text-blue-700 transition-colors">{kb.name}</h3>