chore: add missing file

Author: SwaggyMacro

FanX/Models/DTO/ChartDataPoint.cs

@@ -0,0 +1,13 @@
+namespace FanX.Models.DTO
+{
+    /// <summary>
+    /// Lightweight DTO for chart data to minimize SignalR payload
+    /// </summary>
+    public class ChartDataPoint
+    {
+        public DateTime T { get; set; }  // Timestamp (short name to reduce JSON size)
+        public double V { get; set; }    // Value/Reading
+        public string? N { get; set; }   // SensorName
+        public string? S { get; set; }   // SensorType
+    }
+}