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

src/Jellyfin.Database/Jellyfin.Database.Implementations/IJellyfinDatabaseProvider.cs

@@ -17,6 +17,12 @@ public interface IJellyfinDatabaseProvider
     /// </summary>
     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>

src/Jellyfin.Database/Jellyfin.Database.Providers.Sqlite/SqliteDatabaseProvider.cs

@@ -39,6 +39,9 @@ public sealed class SqliteDatabaseProvider : IJellyfinDatabaseProvider
     /// <inheritdoc/>
     public IDbContextFactory<JellyfinDbContext>? DbContextFactory { get; set; }
 
+    /// <inheritdoc/>
+    public IDescendantQueryProvider DescendantQueryProvider { get; } = new SqliteDescendantQueryProvider();
+
     /// <inheritdoc/>
     public void Initialise(DbContextOptionsBuilder options, DatabaseConfigurationOptions databaseConfiguration)
     {