feat: Pass backgroundSize in playlist API response

Controllers/DisplayController.cs

@@ -50,6 +50,7 @@ public class DisplayController : Controller
             icsSource = ds.Slide.IcsSource,
             backgroundColor = ds.Slide.BackgroundColor,
             backgroundImage = ds.Slide.BackgroundImage,
+            backgroundSize = ds.Slide.BackgroundSize,
             customCss = ds.Slide.CustomCss,
             duration = ds.DurationSeconds,
             updatedAt = ds.Slide.UpdatedAt