fix: VLM 食物识别全链路修复 + 用药 Agent Prompt 优化

- 图片上传自动压缩（max 860px, JPEG Q65），解决大图超 API 129KB 限制
- 修复 VLM 传 file:// 本地路径 Bug，改为 base64 data URL
- VLM Prompt 优化为中文食堂场景，附带常见中餐热量参考
- 千问 API 错误信息透传，方便调试
- 用药 Agent Prompt 加查询规则：先调 manage_medication 再回答
- 新增 System.Drawing.Common 依赖用于服务端图片压缩

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

@@ -129,7 +129,11 @@ public sealed class QwenVisionClient(HttpClient http, IConfiguration config)
         var json = JsonSerializer.Serialize(request, _jsonOptions);
         var content = new StringContent(json, Encoding.UTF8, "application/json");
         var response = await _http.PostAsync("chat/completions", content, ct);
-        response.EnsureSuccessStatusCode();
+        if (!response.IsSuccessStatusCode)
+        {
+            var errorBody = await response.Content.ReadAsStringAsync(ct);
+            throw new HttpRequestException($"VLM API {response.StatusCode}: {errorBody}");
+        }
         var body = await response.Content.ReadAsStringAsync(ct);
         return JsonSerializer.Deserialize<ChatCompletionResponse>(body, _jsonOptions)!;
     }