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/Endpoints/ai_chat_endpoints.cs

@@ -33,6 +33,7 @@ public static class AiChatEndpoints
             HttpContext http,
             DeepSeekClient llmClient,
             PromptManager promptManager,
+            FastGptKnowledgeClient knowledgeClient,
             IAiToolExecutionService toolExecution,
             IAiWriteConfirmationStore confirmations,
             IAiConversationService conversations,
@@ -110,9 +111,24 @@ public static class AiChatEndpoints
             var systemPrompt = promptManager.GetSystemPrompt(parsedType);
             var patientContext = await patientContexts.BuildAsync(userId.Value, ct);
 
+            // ── RAG 知识库检索 ──
+            // 仅对需要医学知识的 Agent 启用检索；记数据/管用药/排运动这些纯工具调用类跳过。
+            // 失败/空结果不阻塞流程，仅作为 prompt 增强。
+            var knowledgeSnippet = "";
+            if (NeedsKnowledgeBase(parsedType) && knowledgeClient.IsEnabled)
+            {
+                knowledgeSnippet = await knowledgeClient.SearchAsync(message, ct);
+            }
+
+            var enhancedSystem = systemPrompt + "\n\n当前患者信息：\n" + patientContext;
+            if (!string.IsNullOrWhiteSpace(knowledgeSnippet))
+            {
+                enhancedSystem += "\n\n知识库参考资料（如与用户问题相关请优先采用）：\n" + knowledgeSnippet;
+            }
+
             var messages = new List<ChatMessage>
             {
-                new() { Role = "system", Content = systemPrompt + "\n\n当前患者信息：\n" + patientContext },
+                new() { Role = "system", Content = enhancedSystem },
             };
 
             // 加载历史对话（最近 10 条）
@@ -303,6 +319,20 @@ public static class AiChatEndpoints
 
     // ── Agent / Tool 调度 ──
 
+    /// <summary>
+    /// 哪些 Agent 需要走 FastGPT 知识库 RAG。
+    /// 纯工具调用类（Health/Medication/Exercise）跳过 RAG，避免拖慢响应。
+    /// </summary>
+    private static bool NeedsKnowledgeBase(AgentType agentType) => agentType switch
+    {
+        AgentType.Default => true,
+        AgentType.Consultation => true,
+        AgentType.Diet => true,
+        AgentType.Report => true,
+        AgentType.Unified => true,
+        _ => false,
+    };
+
     private static List<ToolDefinition> GetToolsForAgent(AgentType agentType) => agentType switch
     {
         AgentType.Health => HealthDataAgentHandler.Tools,