refactor(backend): incremental architecture evolution (Task 19)

- Add common/ layer for shared capabilities (LLM, RAG, document, middleware)
- Add legacy/ layer for existing business code
- Move files to new structure (controllers, routes, services)
- Update index.ts for new route registration
- System remains fully functional

backend/src/config/env.ts

@@ -27,8 +27,15 @@ export const config = {
 
   // LLM API配置
   deepseekApiKey: process.env.DEEPSEEK_API_KEY || '',
+  deepseekBaseUrl: process.env.DEEPSEEK_BASE_URL || 'https://api.deepseek.com',
+  
   dashscopeApiKey: process.env.DASHSCOPE_API_KEY || '',  // 用于Qwen模型
   geminiApiKey: process.env.GEMINI_API_KEY || '',
+  
+  // CloseAI配置（代理OpenAI和Claude）
+  closeaiApiKey: process.env.CLOSEAI_API_KEY || '',
+  closeaiOpenaiBaseUrl: process.env.CLOSEAI_OPENAI_BASE_URL || 'https://api.openai-proxy.org/v1',
+  closeaiClaudeBaseUrl: process.env.CLOSEAI_CLAUDE_BASE_URL || 'https://api.openai-proxy.org/anthropic',
 
   // Dify配置
   difyApiKey: process.env.DIFY_API_KEY || '',