fix: 添加缺失的 StrategySignalResponse DTO

- 新增 StrategySignalResponse 类（策略信号响应）
- 新增 SignalAction 类（信号行动建议）
- 修正 PortfolioFacade 属性名为 PascalCase

AssetManager.Models/DTOs/StrategyDTO.cs

@@ -130,3 +130,45 @@ public class StrategyListItemDto
     public DateTime CreatedAt { get; set; }
     public DateTime UpdatedAt { get; set; }
 }
+
+/// <summary>
+/// 策略信号响应
+/// </summary>
+public class StrategySignalResponse
+{
+    /// <summary>
+    /// 信号类型：buy / sell / hold / rebalance
+    /// </summary>
+    public string? Signal { get; set; }
+    
+    /// <summary>
+    /// 信号原因说明
+    /// </summary>
+    public string? Reason { get; set; }
+    
+    /// <summary>
+    /// 建议操作（可选）
+    /// </summary>
+    public List<SignalAction>? Actions { get; set; }
+}
+
+/// <summary>
+/// 信号行动建议
+/// </summary>
+public class SignalAction
+{
+    /// <summary>
+    /// 股票代码
+    /// </summary>
+    public string? Symbol { get; set; }
+    
+    /// <summary>
+    /// 行动类型：buy / sell
+    /// </summary>
+    public string? Action { get; set; }
+    
+    /// <summary>
+    /// 目标权重或数量
+    /// </summary>
+    public double? Target { get; set; }
+}

AssetManager.Services/PortfolioFacade.cs

@@ -98,14 +98,14 @@ public class PortfolioFacade : IPortfolioFacade
         var portfolio = _databaseService.GetPortfolioById(portfolioId, userId);
         if (portfolio == null || string.IsNullOrEmpty(portfolio.StrategyId))
         {
-            return new StrategySignalResponse { signal = "hold", reason = "未绑定策略" };
+            return new StrategySignalResponse { Signal = "hold", Reason = "未绑定策略" };
         }
         
         // 获取策略
         var strategy = await _strategyService.GetStrategyByIdAsync(portfolio.StrategyId, userId);
         if (strategy == null)
         {
-            return new StrategySignalResponse { signal = "hold", reason = "策略不存在" };
+            return new StrategySignalResponse { Signal = "hold", Reason = "策略不存在" };
         }
         
         // 计算信号