Add LinkedChildren database table for normalized relationships

Introduces a new database table to store linked child relationships for
boxsets, playlists, and video alternate versions. This replaces the
JSON-serialized Data column approach with a proper relational structure.

- Add LinkedChildEntity and LinkedChildType enum
- Add entity configuration with proper foreign keys
- Add EF Core migration for SQLite

src/Jellyfin.Database/Jellyfin.Database.Implementations/Entities/BaseItemEntity.cs

@@ -178,6 +178,16 @@ public class BaseItemEntity
 
     public ICollection<BaseItemImageInfo>? Images { get; set; }
 
+    /// <summary>
+    /// Gets or sets the linked children (for BoxSets, Playlists, etc.).
+    /// </summary>
+    public ICollection<LinkedChildEntity>? LinkedChildEntities { get; set; }
+
+    /// <summary>
+    /// Gets or sets the items this entity is linked to as a child.
+    /// </summary>
+    public ICollection<LinkedChildEntity>? LinkedChildOfEntities { get; set; }
+
     // those are references to __LOCAL__ ids not DB ids ... TODO: Bring the whole folder structure into the DB
     // public ICollection<BaseItemEntity>? SeriesEpisodes { get; set; }
     // public BaseItemEntity? Series { get; set; }

src/Jellyfin.Database/Jellyfin.Database.Implementations/Entities/LinkedChildEntity.cs

@@ -0,0 +1,39 @@
+using System;
+
+namespace Jellyfin.Database.Implementations.Entities;
+
+/// <summary>
+/// Represents a linked child relationship between items (e.g., BoxSet to Movies, Playlist to tracks).
+/// </summary>
+public class LinkedChildEntity
+{
+    /// <summary>
+    /// Gets or sets the parent item ID (BoxSet, Playlist, etc.).
+    /// </summary>
+    public required Guid ParentId { get; set; }
+
+    /// <summary>
+    /// Gets or sets the child item ID.
+    /// </summary>
+    public required Guid ChildId { get; set; }
+
+    /// <summary>
+    /// Gets or sets the type of linked child (Manual or Shortcut).
+    /// </summary>
+    public required LinkedChildType ChildType { get; set; }
+
+    /// <summary>
+    /// Gets or sets the sort order.
+    /// </summary>
+    public int? SortOrder { get; set; }
+
+    /// <summary>
+    /// Gets or sets the parent item navigation property.
+    /// </summary>
+    public BaseItemEntity? Parent { get; set; }
+
+    /// <summary>
+    /// Gets or sets the child item navigation property.
+    /// </summary>
+    public BaseItemEntity? Child { get; set; }
+}

src/Jellyfin.Database/Jellyfin.Database.Implementations/Entities/LinkedChildType.cs

@@ -0,0 +1,27 @@
+namespace Jellyfin.Database.Implementations.Entities;
+
+/// <summary>
+/// The linked child type.
+/// </summary>
+public enum LinkedChildType
+{
+    /// <summary>
+    /// Manually linked child.
+    /// </summary>
+    Manual = 0,
+
+    /// <summary>
+    /// Shortcut linked child.
+    /// </summary>
+    Shortcut = 1,
+
+    /// <summary>
+    /// Local alternate version (same item, different file path).
+    /// </summary>
+    LocalAlternateVersion = 2,
+
+    /// <summary>
+    /// Linked alternate version (different item ID).
+    /// </summary>
+    LinkedAlternateVersion = 3
+}

src/Jellyfin.Database/Jellyfin.Database.Implementations/JellyfinDbContext.cs

@@ -143,6 +143,11 @@ public class JellyfinDbContext(DbContextOptions<JellyfinDbContext> options, ILog
     /// </summary>
     public DbSet<PeopleBaseItemMap> PeopleBaseItemMap => Set<PeopleBaseItemMap>();
 
+    /// <summary>
+    /// Gets the <see cref="DbSet{TEntity}"/> containing linked children relationships.
+    /// </summary>
+    public DbSet<LinkedChildEntity> LinkedChildren => Set<LinkedChildEntity>();
+
     /// <summary>
     /// Gets the <see cref="DbSet{TEntity}"/> containing the referenced Providers with ids.
     /// </summary>

src/Jellyfin.Database/Jellyfin.Database.Implementations/ModelConfiguration/LinkedChildConfiguration.cs

@@ -0,0 +1,33 @@
+using Jellyfin.Database.Implementations.Entities;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
+
+namespace Jellyfin.Database.Implementations.ModelConfiguration;
+
+/// <summary>
+/// LinkedChildEntity configuration.
+/// </summary>
+public class LinkedChildConfiguration : IEntityTypeConfiguration<LinkedChildEntity>
+{
+    /// <inheritdoc/>
+    public void Configure(EntityTypeBuilder<LinkedChildEntity> builder)
+    {
+        builder.ToTable("LinkedChildren");
+        builder.HasKey(e => new { e.ParentId, e.ChildId });
+        builder.HasIndex(e => e.ParentId);
+        builder.HasIndex(e => e.ChildId);
+        builder.HasIndex(e => new { e.ParentId, e.SortOrder });
+        builder.HasIndex(e => new { e.ParentId, e.ChildType });
+        builder.HasIndex(e => new { e.ChildId, e.ChildType });
+
+        builder.HasOne(e => e.Parent)
+            .WithMany(e => e.LinkedChildEntities)
+            .HasForeignKey(e => e.ParentId)
+            .OnDelete(DeleteBehavior.NoAction);
+
+        builder.HasOne(e => e.Child)
+            .WithMany(e => e.LinkedChildOfEntities)
+            .HasForeignKey(e => e.ChildId)
+            .OnDelete(DeleteBehavior.NoAction);
+    }
+}