feat: 接入FastGPT知识库RAG检索，AI回答前先查医学资料

- 新增 FastGptKnowledgeClient：调用 searchTest 接口检索知识库
- ai_chat_endpoints: 调 DeepSeek 之前先检索，把相关片段拼到 system prompt
- 5个智能体（default/consultation/diet/report/unified）启用 RAG
- 3个纯工具调用智能体（health/medication/exercise）保持原样
- .env: 新增 FastGPT 配置（API Key、datasetId、相似度等）
- api_client: 更新默认 baseUrl 适配新网络环境

backend/src/Health.WebApi/Program.cs

@@ -162,6 +162,12 @@ builder.Services.AddHttpClient<VisionClient>(client =>
     client.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Bearer", builder.Configuration["VLM_API_KEY"] ?? "");
     client.Timeout = TimeSpan.FromSeconds(120);
 });
+builder.Services.AddHttpClient<FastGptKnowledgeClient>(client =>
+{
+    client.BaseAddress = new Uri((builder.Configuration["FASTGPT_BASE_URL"] ?? "https://cloud.fastgpt.cn/api").TrimEnd('/') + "/");
+    client.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Bearer", builder.Configuration["FASTGPT_API_KEY"] ?? "");
+    client.Timeout = TimeSpan.FromSeconds(15);
+});
 
 // ---- 后台服务 ----
 builder.Services.AddHostedService<MedicationReminderService>();