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.Infrastructure/AI/fastgpt_knowledge_client.cs

@@ -0,0 +1,71 @@
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.Logging;
+
+namespace Health.Infrastructure.AI;
+
+/// <summary>
+/// FastGPT 知识库检索客户端（仅做 RAG 检索，不参与生成）。
+/// 我们后端的 DeepSeek 依旧负责对话生成与工具调用；
+/// 这里只把检索到的医学知识片段拼回 system prompt，提升回答准确度。
+/// </summary>
+public sealed class FastGptKnowledgeClient(HttpClient http, IConfiguration config, ILogger<FastGptKnowledgeClient> logger)
+{
+    private readonly HttpClient _http = http;
+    private readonly string _datasetId = config["FASTGPT_DATASET_ID"] ?? "";
+    private readonly float _similarity = float.TryParse(config["FASTGPT_SIMILARITY"], out var s) ? s : 0.4f;
+    private readonly int _limit = int.TryParse(config["FASTGPT_LIMIT"], out var l) ? l : 3000;
+    private readonly ILogger<FastGptKnowledgeClient> _logger = logger;
+    private readonly JsonSerializerOptions _jsonOptions = new() { PropertyNameCaseInsensitive = true };
+
+    public bool IsEnabled => !string.IsNullOrWhiteSpace(_datasetId);
+
+    /// <summary>
+    /// 按用户问题检索知识库，返回最相关的文档片段。
+    /// 失败或未配置时返回空字符串，调用方应当容错——RAG 只是增强，不是必需。
+    /// </summary>
+    public async Task<string> SearchAsync(string question, CancellationToken ct = default)
+    {
+        if (!IsEnabled || string.IsNullOrWhiteSpace(question)) return "";
+
+        try
+        {
+            var payload = new
+            {
+                datasetId = _datasetId,
+                text = question,
+                limit = _limit,
+                similarity = _similarity,
+                searchMode = "embedding",
+            };
+            var json = JsonSerializer.Serialize(payload);
+            var content = new StringContent(json, Encoding.UTF8, "application/json");
+            using var response = await _http.PostAsync("core/dataset/searchTest", content, ct);
+            if (!response.IsSuccessStatusCode)
+            {
+                _logger.LogWarning("FastGPT 检索失败: {Status}", response.StatusCode);
+                return "";
+            }
+
+            var body = await response.Content.ReadAsStringAsync(ct);
+            using var doc = JsonDocument.Parse(body);
+            if (!doc.RootElement.TryGetProperty("data", out var data)) return "";
+            if (!data.TryGetProperty("list", out var list) || list.ValueKind != JsonValueKind.Array) return "";
+
+            var snippets = new List<string>();
+            foreach (var item in list.EnumerateArray())
+            {
+                var q = item.TryGetProperty("q", out var qEl) ? qEl.GetString() ?? "" : "";
+                var a = item.TryGetProperty("a", out var aEl) ? aEl.GetString() ?? "" : "";
+                if (string.IsNullOrWhiteSpace(q) && string.IsNullOrWhiteSpace(a)) continue;
+                snippets.Add(string.IsNullOrWhiteSpace(a) ? q : $"{q}：{a}");
+            }
+
+            return snippets.Count == 0 ? "" : string.Join("\n", snippets);
+        }
+        catch (Exception ex)
+        {
+            _logger.LogWarning(ex, "FastGPT 检索异常，已忽略");
+            return "";
+        }
+    }
+}