feat: add IIntroProvider for Wholphin/cinema-mode compatibility

Registers TrailerIntroProvider as IIntroProvider. Queries fake-movie
items in the output folder and returns their local trailer extras
(LocalTrailers) as IntroInfo — mirroring jellyfin-plugin-cinemamode's
proven pattern so Wholphin plays the actual trailer, not the 3-second
black placeholder.

Recursion guard: items whose path starts with the output folder are
excluded from intro injection, so Wholphin's getIntros call on the
intro item itself returns empty. Server-scoped query bypasses library
visibility restrictions so hidden trailer libraries still work.

Adds TrailersPerMovie config option (default 1, 0 = disabled).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Jellyfin.Plugin.CinemaTrailers4Jellyfins/Services/TrailerIntroProvider.cs

@@ -0,0 +1,85 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Threading.Tasks;
+using Jellyfin.Data.Enums;
+using Jellyfin.Database.Implementations.Entities;
+using MediaBrowser.Controller.Entities;
+using MediaBrowser.Controller.Entities.Movies;
+using MediaBrowser.Controller.Library;
+using Microsoft.Extensions.Logging;
+
+namespace Jellyfin.Plugin.CinemaTrailers4Jellyfins.Services
+{
+    public class TrailerIntroProvider : IIntroProvider
+    {
+        private static readonly Random _rng = new();
+        private readonly ILibraryManager _libraryManager;
+        private readonly ILogger<TrailerIntroProvider> _logger;
+
+        public string Name => "CinemaTrailers4Jellyfins";
+
+        public TrailerIntroProvider(ILibraryManager libraryManager, ILogger<TrailerIntroProvider> logger)
+        {
+            _libraryManager = libraryManager;
+            _logger = logger;
+        }
+
+        public Task<IEnumerable<IntroInfo>> GetIntros(BaseItem item, User user)
+        {
+            var config = Plugin.Instance?.Configuration;
+            if (config == null || config.TrailersPerMovie <= 0)
+                return Task.FromResult(Enumerable.Empty<IntroInfo>());
+
+            if (item is not Movie)
+                return Task.FromResult(Enumerable.Empty<IntroInfo>());
+
+            var outputFolder = config.DownloadFolder?.TrimEnd(
+                Path.DirectorySeparatorChar, Path.AltDirectorySeparatorChar);
+            if (string.IsNullOrEmpty(outputFolder))
+                return Task.FromResult(Enumerable.Empty<IntroInfo>());
+
+            // Break potential recursion: don't inject intros before items that live inside
+            // our output folder. This covers both the fake-movie files and their trailer
+            // extras, so Wholphin can't trigger a getIntros loop on the intro item itself.
+            if (item.Path?.StartsWith(outputFolder, StringComparison.OrdinalIgnoreCase) == true)
+                return Task.FromResult(Enumerable.Empty<IntroInfo>());
+
+            // Server-scoped query (no User parameter) so library visibility settings don't
+            // prevent trailer discovery even when the output library is hidden from users.
+            var fakeMovies = _libraryManager
+                .GetItemList(new InternalItemsQuery
+                {
+                    IncludeItemTypes = new[] { BaseItemKind.Movie },
+                    Recursive = true,
+                })
+                .OfType<Movie>()
+                .Where(m =>
+                    m.Path?.StartsWith(outputFolder, StringComparison.OrdinalIgnoreCase) == true
+                    && m.LocalTrailers.Count > 0)
+                .ToList();
+
+            if (fakeMovies.Count == 0)
+            {
+                _logger.LogDebug(
+                    "|CinemaTrailers4Jellyfins| No indexed trailers found in {Folder}. "
+                    + "Ensure the output folder is added as a Jellyfin Movies library and scanned.",
+                    outputFolder);
+                return Task.FromResult(Enumerable.Empty<IntroInfo>());
+            }
+
+            var selected = fakeMovies
+                .OrderBy(_ => _rng.Next())
+                .Take(config.TrailersPerMovie)
+                .Select(m =>
+                {
+                    var t = m.LocalTrailers.ElementAt(_rng.Next(m.LocalTrailers.Count));
+                    return new IntroInfo { ItemId = t.Id, Path = t.Path };
+                })
+                .ToList();
+
+            return Task.FromResult<IEnumerable<IntroInfo>>(selected);
+        }
+    }
+}