Complete LinkedChildren integration and batch DTO optimizations

This commit integrates remaining performance changes:

- Add batch user data fetching in DtoService to reduce N+1 queries
- Add GetNextUpEpisodesBatch in TVSeriesManager for efficient batch retrieval
- Update Video/Movie/BoxSet to use LibraryManager for alternate versions
- Transition LinkedChild to use ItemId instead of Path (obsolete Path/LibraryItemId)
- Update providers and controllers for LinkedChildren-based references
- Add NextUpEpisodeBatchResult for batched episode queries
- Integrate IDescendantQueryProvider in SqliteDatabaseProvider

2 files changed, 9 insertions, 0 deletions

diff --git a/src/Jellyfin.Database/Jellyfin.Database.Implementations/IJellyfinDatabaseProvider.cs b/src/Jellyfin.Database/Jellyfin.Database.Implementations/IJellyfinDatabaseProvider.cs
index 27dbeaba6a..f52a68c684 100644
--- a/src/Jellyfin.Database/Jellyfin.Database.Implementations/IJellyfinDatabaseProvider.cs
+++ b/src/Jellyfin.Database/Jellyfin.Database.Implementations/IJellyfinDatabaseProvider.cs
@@ -18,6 +18,12 @@ public interface IJellyfinDatabaseProvider
     IDbContextFactory<JellyfinDbContext>? DbContextFactory { get; set; }
 
     /// <summary>
+    /// Gets the descendant query provider for this database type.
+    /// Used for recursive CTE queries to find all descendants of an item.
+    /// </summary>
+    IDescendantQueryProvider DescendantQueryProvider { get; }
+
+    /// <summary>
     /// Initialises jellyfins EFCore database access.
     /// </summary>
     /// <param name="options">The EFCore database options.</param>
diff --git a/src/Jellyfin.Database/Jellyfin.Database.Providers.Sqlite/SqliteDatabaseProvider.cs b/src/Jellyfin.Database/Jellyfin.Database.Providers.Sqlite/SqliteDatabaseProvider.cs
index da63df8e29..94c470e6cb 100644
--- a/src/Jellyfin.Database/Jellyfin.Database.Providers.Sqlite/SqliteDatabaseProvider.cs
+++ b/src/Jellyfin.Database/Jellyfin.Database.Providers.Sqlite/SqliteDatabaseProvider.cs
@@ -40,6 +40,9 @@ public sealed class SqliteDatabaseProvider : IJellyfinDatabaseProvider
     public IDbContextFactory<JellyfinDbContext>? DbContextFactory { get; set; }
 
     /// <inheritdoc/>
+    public IDescendantQueryProvider DescendantQueryProvider { get; } = new SqliteDescendantQueryProvider();
+
+    /// <inheritdoc/>
     public void Initialise(DbContextOptionsBuilder options, DatabaseConfigurationOptions databaseConfiguration)
     {
         static T? GetOption<T>(ICollection<CustomDatabaseOption>? options, string key, Func<string, T> converter, Func<T>? defaultValue = null)