Fix playlist entries being lost on migration and library scans

MediaBrowser.Controller/Entities/Folder.cs

@@ -43,11 +43,7 @@ namespace MediaBrowser.Controller.Entities
     public class Folder : BaseItem
     {
         private IEnumerable<BaseItem> _children;
-
-        public Folder()
-        {
-            LinkedChildren = Array.Empty<LinkedChild>();
-        }
+        private LinkedChild[] _linkedChildren = [];
 
         public static IUserViewManager UserViewManager { get; set; }
 
@@ -63,7 +59,27 @@ namespace MediaBrowser.Controller.Entities
         /// Gets or sets the linked children.
         /// </summary>
         [JsonIgnore]
-        public LinkedChild[] LinkedChildren { get; set; }
+        public LinkedChild[] LinkedChildren
+        {
+            get => _linkedChildren;
+            set
+            {
+                _linkedChildren = value;
+
+                // Assigning the collection means the caller knows the complete set of links.
+                LinkedChildrenLoaded = true;
+            }
+        }
+
+        /// <summary>
+        /// Gets a value indicating whether <see cref="LinkedChildren"/> holds the stored set of links.
+        /// </summary>
+        /// <remarks>
+        /// An unloaded instance carries an empty array that means "unknown", not "no children" —
+        /// persisting it would delete every link the item has.
+        /// </remarks>
+        [JsonIgnore]
+        public bool LinkedChildrenLoaded { get; private set; }
 
         [JsonIgnore]
         public DateTime? DateLastMediaAdded { get; set; }