feat(wechat): Add default template data builder for notification templates

- Add BuildDefaultTemplateData method to construct fallback template data
- Map notification fields to WeChat template placeholders (first, keyword1-3, remark)
- Provide default template structure when database field mapping is not configured
- Ensures template messages can be sent with sensible defaults as safety net

server/src/XiangYi.Application/Services/NotificationService.cs

@@ -630,6 +630,22 @@ public class NotificationService : INotificationService
         }
     }
 
+    /// <summary>
+    /// 构建默认模板数据（兜底，数据库未配置字段映射时使用）
+    /// </summary>
+    private static Dictionary<string, TemplateDataItem> BuildDefaultTemplateData(
+        NotificationTemplateType templateType, string title, string name, string content, string time)
+    {
+        return new Dictionary<string, TemplateDataItem>
+        {
+            ["first"] = new TemplateDataItem { Value = title },
+            ["keyword1"] = new TemplateDataItem { Value = name },
+            ["keyword2"] = new TemplateDataItem { Value = content },
+            ["keyword3"] = new TemplateDataItem { Value = time },
+            ["remark"] = new TemplateDataItem { Value = "点击查看详情" }
+        };
+    }
+
     #endregion