fix: VLM 参数优化 - temperature 0.7, top_p 0.8, 指令放 system+user

- VisionAsync 新增 Temperature=0.7, TopP=0.8
- system prompt 用专业营养识别指令
- userText 用简短"请看图识别食物"配合图片
- 修复重复 prompt 导致 VLM 误读文本的 bug

backend/src/Health.Infrastructure/AI/ai_clients.cs

@@ -124,6 +124,7 @@ public sealed class VisionClient(HttpClient http, IConfiguration config)
         var request = new ChatCompletionRequest
         {
             Model = _model, Messages = messages, MaxTokens = maxTokens, Stream = false,
+            Temperature = 0.7f, TopP = 0.8f,
         };
 
         var json = JsonSerializer.Serialize(request, _jsonOptions);