feat: Add BackgroundSize property to Slide model

Models/Slide.cs

@@ -40,6 +40,10 @@ public class Slide
     [MaxLength(500)]
     public string? BackgroundImage { get; set; }
 
+    /// <summary>Background image sizing: cover, contain, fill, scale-down, auto</summary>
+    [MaxLength(20)]
+    public string BackgroundSize { get; set; } = "cover";
+
     public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
     public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;