fix: add /openai/v1/chat/completions route support

- Register unifiedRoutes under /openai prefix to enable /openai/v1/chat/completions
- Reuse existing intelligent routing logic from unified.js (no code duplication)
- Keep existing Codex API routes (/openai/responses, /openai/v1/responses) unchanged

Benefits:
- Fixes 404 error for /openai/v1/chat/completions endpoint
- Provides consistent API experience across /api and /openai prefixes
- Automatically routes to correct backend (Claude/OpenAI/Gemini) based on model

Tested:
- ✅ /openai/v1/chat/completions now returns authentication error (route works)
- ✅ /api/v1/chat/completions continues to work
- ✅ Existing Codex routes remain functional

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/app.js

@@ -269,7 +269,8 @@ class Application {
       this.app.use('/gemini', geminiRoutes) // 保留原有路径以保持向后兼容
       this.app.use('/openai/gemini', openaiGeminiRoutes)
       this.app.use('/openai/claude', openaiClaudeRoutes)
-      this.app.use('/openai', openaiRoutes)
+      this.app.use('/openai', unifiedRoutes) // 复用统一智能路由，支持 /openai/v1/chat/completions
+      this.app.use('/openai', openaiRoutes) // Codex API 路由（/openai/responses, /openai/v1/responses）
       // Droid 路由：支持多种 Factory.ai 端点
       this.app.use('/droid', droidRoutes) // Droid (Factory.ai) API 转发
       this.app.use('/azure', azureOpenaiRoutes)