Add folder-aware filter extensions and descendant query provider

- Add FolderAwareFilterExtensions for LinkedChildren-based filtering
- Add IDescendantQueryProvider interface for database-specific queries
- Add MatchCriteria classes for folder filtering
- Add SqliteDescendantQueryProvider implementation

Jellyfin.Server.Implementations/Item/FolderAwareFilterExtensions.cs

@@ -0,0 +1,65 @@
+using System;
+using System.Linq;
+using System.Linq.Expressions;
+using Jellyfin.Database.Implementations;
+using Jellyfin.Database.Implementations.Entities;
+
+namespace Jellyfin.Server.Implementations.Item;
+
+/// <summary>
+/// Extension methods for applying folder-aware filters that check items and their descendants.
+/// </summary>
+internal static class FolderAwareFilterExtensions
+{
+    /// <summary>
+    /// Filters items where either the item matches the condition (for non-folders)
+    /// or any descendant matches (for folders). Uses reverse traversal through AncestorIds.
+    /// </summary>
+    /// <param name="query">The query to filter.</param>
+    /// <param name="context">The database context.</param>
+    /// <param name="condition">The condition to check on BaseItemEntity.</param>
+    /// <returns>Filtered query.</returns>
+    public static IQueryable<BaseItemEntity> WhereItemOrDescendantMatches(
+        this IQueryable<BaseItemEntity> query,
+        JellyfinDbContext context,
+        Expression<Func<BaseItemEntity, bool>> condition)
+    {
+        var matchingIds = context.BaseItems.Where(condition).Select(b => b.Id);
+        var foldersWithMatchingDescendants = context.AncestorIds
+            .Where(a => matchingIds.Contains(a.ItemId))
+            .Select(a => a.ParentItemId)
+            .Union(context.LinkedChildren
+                .Where(lc => matchingIds.Contains(lc.ChildId))
+                .Select(lc => lc.ParentId));
+
+        return query.Where(e =>
+            matchingIds.Contains(e.Id)
+            || foldersWithMatchingDescendants.Contains(e.Id));
+    }
+
+    /// <summary>
+    /// Filters items where neither the item matches the condition (for non-folders)
+    /// nor any descendant matches (for folders). Uses reverse traversal for infinite depth.
+    /// </summary>
+    /// <param name="query">The query to filter.</param>
+    /// <param name="context">The database context.</param>
+    /// <param name="condition">The condition that should NOT match.</param>
+    /// <returns>Filtered query.</returns>
+    public static IQueryable<BaseItemEntity> WhereNeitherItemNorDescendantMatches(
+        this IQueryable<BaseItemEntity> query,
+        JellyfinDbContext context,
+        Expression<Func<BaseItemEntity, bool>> condition)
+    {
+        var matchingIds = context.BaseItems.Where(condition).Select(b => b.Id);
+        var foldersWithMatchingDescendants = context.AncestorIds
+            .Where(a => matchingIds.Contains(a.ItemId))
+            .Select(a => a.ParentItemId)
+            .Union(context.LinkedChildren
+                .Where(lc => matchingIds.Contains(lc.ChildId))
+                .Select(lc => lc.ParentId));
+
+        return query.Where(e =>
+            !matchingIds.Contains(e.Id)
+            && !foldersWithMatchingDescendants.Contains(e.Id));
+    }
+}