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

src/Jellyfin.Database/Jellyfin.Database.Implementations/IDescendantQueryProvider.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Linq;
+using Jellyfin.Database.Implementations.MatchCriteria;
+
+namespace Jellyfin.Database.Implementations;
+
+/// <summary>
+/// Provider interface for descendant queries using recursive CTEs.
+/// Each database provider implements this with provider-specific SQL.
+/// </summary>
+public interface IDescendantQueryProvider
+{
+    /// <summary>
+    /// Gets a queryable of all descendant IDs for a parent item.
+    /// Uses recursive CTE to traverse AncestorIds and LinkedChildren infinitely.
+    /// </summary>
+    /// <param name="context">Database context.</param>
+    /// <param name="parentId">Parent item ID.</param>
+    /// <returns>Queryable of descendant item IDs.</returns>
+    IQueryable<Guid> GetAllDescendantIds(JellyfinDbContext context, Guid parentId);
+
+    /// <summary>
+    /// Gets a queryable of all folder IDs that have any descendant matching the specified criteria.
+    /// Uses recursive CTE for infinite depth traversal. Can be used in LINQ .Contains() expressions.
+    /// </summary>
+    /// <param name="context">Database context.</param>
+    /// <param name="criteria">The matching criteria to apply.</param>
+    /// <returns>Queryable of folder IDs.</returns>
+    IQueryable<Guid> GetFolderIdsMatching(JellyfinDbContext context, FolderMatchCriteria criteria);
+}

src/Jellyfin.Database/Jellyfin.Database.Implementations/MatchCriteria/FolderMatchCriteria.cs

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+namespace Jellyfin.Database.Implementations.MatchCriteria;
+
+/// <summary>
+/// Base type for folder matching criteria using discriminated union pattern.
+/// </summary>
+public abstract record FolderMatchCriteria;

src/Jellyfin.Database/Jellyfin.Database.Implementations/MatchCriteria/HasChapterImages.cs

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+namespace Jellyfin.Database.Implementations.MatchCriteria;
+
+/// <summary>
+/// Matches folders containing descendants with chapter images.
+/// </summary>
+public sealed record HasChapterImages : FolderMatchCriteria;

src/Jellyfin.Database/Jellyfin.Database.Implementations/MatchCriteria/HasMediaStreamType.cs

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+using Jellyfin.Database.Implementations.Entities;
+
+namespace Jellyfin.Database.Implementations.MatchCriteria;
+
+/// <summary>
+/// Matches folders containing descendants with a specific media stream type and language.
+/// </summary>
+/// <param name="StreamType">The type of media stream to match (Audio, Subtitle, etc.).</param>
+/// <param name="Language">The language to match.</param>
+/// <param name="IsExternal">If not null, filters by internal (false) or external (true) streams. Only applicable to subtitles.</param>
+public sealed record HasMediaStreamType(
+    MediaStreamTypeEntity StreamType,
+    string Language,
+    bool? IsExternal = null) : FolderMatchCriteria;

src/Jellyfin.Database/Jellyfin.Database.Implementations/MatchCriteria/HasSubtitles.cs

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+namespace Jellyfin.Database.Implementations.MatchCriteria;
+
+/// <summary>
+/// Matches folders containing descendants with subtitles.
+/// </summary>
+public sealed record HasSubtitles : FolderMatchCriteria;