feat(album): add cover picture functionality to albums and enhance album management API

Models/DataBase/Album.cs

@@ -1,4 +1,5 @@
 using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
 
 namespace Foxel.Models.DataBase;
 
@@ -14,5 +15,9 @@ public class Album : BaseModel
     [Required]
     public User User { get; set; }
 
+    public int? CoverPictureId { get; set; }
+    [ForeignKey("CoverPictureId")]
+    public Picture? CoverPicture { get; set; }
+
     public ICollection<Picture>? Pictures { get; set; }
 }