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)

backend/src/modules/pkb/controllers/knowledgeBaseController.ts

@@ -47,9 +47,19 @@ export async function createKnowledgeBase(
     });
   } catch (error: any) {
     console.error('Failed to create knowledge base:', error);
+    
+    // 处理配额超限错误
+    if (error.code === 'QUOTA_EXCEEDED') {
+      return reply.status(400).send({
+        success: false,
+        code: 'QUOTA_EXCEEDED',
+        message: error.message,
+      });
+    }
+
     return reply.status(500).send({
       success: false,
-      message: error.message || 'Failed to create knowledge base',
+      message: error.message || '创建知识库失败，请稍后重试',
     });
   }
 }