fix: AI Chat LLM 从已失效 api.qnaigc.com 切换到 OpenCode Zen 免费 API

- 数据库 AIModel 记录从 meituan/longcat-flash-lite → deepseek-v4-flash-free
- API 端点 https://api.qnaigc.com → https://opencode.ai/zen/v1/chat/completions
- aiChat.js 支持免 Authorization 的免费 API（apiKey 为 sk-dummy 时跳过）
- .env 同步更新

server/src/routes/aiChat.js

@@ -91,22 +91,24 @@ router.post('/ask', userAuthMiddleware, async (req, res) => {
       modelConfig = { temperature: 0.7, maxTokens: 800, topP: 1 }
     }
     
-    // 检查API配置
-    if (!apiKey) {
-      logger.error('AI API Key未配置')
-      return ApiResponse.error(res, 'AI服务暂时不可用', 500)
-    }
+    // 检查API配置（opencode.ai免费API无需key）
+    const needsAuth = apiKey && apiKey !== 'sk-dummy' && apiKey !== '***'
     
     // 构建系统提示词
     const systemPrompt = getSystemPromptByDimension(dimension)
     
+    // 构建请求头
+    const headers = {
+      'Content-Type': 'application/json'
+    }
+    if (needsAuth) {
+      headers['Authorization'] = `Bearer ${apiKey}`
+    }
+    
     // 调用AI API
     const response = await fetch(apiUrl, {
       method: 'POST',
-      headers: {
-        'Content-Type': 'application/json',
-        'Authorization': `Bearer ${apiKey}`
-      },
+      headers,
       body: JSON.stringify({
         model: modelId,
         messages: [