feat: Recently Added Guard plugin for Jellyfin 12.2

Keeps Radarr/Sonarr quality upgrades out of "Recently Added" by keeping
a ledger of the first date added per logical medium (movie or episode,
identified by provider ids and the series) and restoring it on upgrades.

- core logic without Jellyfin types (Reconciler, MediaKeys, GuardProcessor)
- thread-safe ledger with atomic, debounced persistence
- event processing via channel and background worker, seed on first start
- scheduled task "Ledger synchronisieren", status API, configuration page
- xUnit tests and a Jellyfin 12.2 integration test
- Gitea workflows: build/test, release with jprm and manifest branch

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Jellyfin.Plugin.RecentlyAddedGuard.Tests/TestSupport.cs

@@ -0,0 +1,136 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+using Jellyfin.Plugin.RecentlyAddedGuard.Core;
+using Jellyfin.Plugin.RecentlyAddedGuard.Ledger;
+using Microsoft.Extensions.Logging.Abstractions;
+using Xunit;
+
+namespace Jellyfin.Plugin.RecentlyAddedGuard.Tests;
+
+internal sealed class ManualTimeProvider : TimeProvider
+{
+    public ManualTimeProvider(DateTimeOffset start)
+    {
+        Now = start;
+    }
+
+    public DateTimeOffset Now { get; set; }
+
+    public override DateTimeOffset GetUtcNow() => Now;
+
+    public void Advance(TimeSpan by) => Now += by;
+}
+
+internal sealed class RecordingDateWriter : IItemDateWriter
+{
+    public List<(Guid ItemId, DateTime Date)> Writes { get; } = [];
+
+    public bool ItemExists { get; set; } = true;
+
+    public Task<bool> TrySetDateCreatedAsync(Guid itemId, DateTime dateCreated, CancellationToken cancellationToken)
+    {
+        if (!ItemExists)
+        {
+            return Task.FromResult(false);
+        }
+
+        Writes.Add((itemId, dateCreated));
+        return Task.FromResult(true);
+    }
+}
+
+/// <summary>
+/// Wires the core classes against a temporary ledger file and a manual clock.
+/// </summary>
+internal sealed class GuardFixture : IDisposable
+{
+    public static readonly DateTimeOffset Start = new(2026, 3, 1, 12, 0, 0, TimeSpan.Zero);
+
+    public GuardFixture(GuardOptions? options = null)
+    {
+        DirectoryPath = Path.Combine(Path.GetTempPath(), "rag-tests", Guid.NewGuid().ToString("N"));
+        Time = new ManualTimeProvider(Start);
+        Options = options ?? GuardOptions.Default;
+        Store = CreateStore();
+        Reconciler = new Reconciler(Store, () => Options, Time, NullLogger<Reconciler>.Instance);
+        Writer = new RecordingDateWriter();
+        Corrections = new CorrectionLog();
+        Processor = new GuardProcessor(Reconciler, Writer, Corrections, () => Options, Time, NullLogger<GuardProcessor>.Instance);
+    }
+
+    public string DirectoryPath { get; }
+
+    public string LedgerPath => Path.Combine(DirectoryPath, LedgerStore.DefaultFileName);
+
+    public ManualTimeProvider Time { get; }
+
+    public GuardOptions Options { get; set; }
+
+    public LedgerStore Store { get; }
+
+    public Reconciler Reconciler { get; }
+
+    public RecordingDateWriter Writer { get; }
+
+    public CorrectionLog Corrections { get; }
+
+    public GuardProcessor Processor { get; }
+
+    public DateTime Now => Time.GetUtcNow().UtcDateTime;
+
+    /// <summary>
+    /// Creates another store on the same file. The save delay is long, so tests control writes via Flush.
+    /// </summary>
+    public LedgerStore CreateStore(TimeSpan? saveDelay = null)
+        => new(LedgerPath, NullLogger<LedgerStore>.Instance, Time, saveDelay ?? TimeSpan.FromHours(1));
+
+    public LedgerEntry SingleEntry()
+    {
+        var entries = Store.GetEntries();
+        Assert.Single(entries);
+        return entries[0];
+    }
+
+    public void Dispose()
+    {
+        Store.Dispose();
+        try
+        {
+            Directory.Delete(DirectoryPath, recursive: true);
+        }
+        catch (DirectoryNotFoundException)
+        {
+        }
+    }
+}
+
+internal static class Media
+{
+    public static MediaIdentity Movie(
+        Guid itemId,
+        DateTime dateCreated,
+        string? tmdb = null,
+        string? imdb = null,
+        string? tvdb = null,
+        string name = "The Matrix (1999)")
+        => new(itemId, MediaKind.Movie, dateCreated, MediaKeys.ForMovie(tmdb, imdb, tvdb), name);
+
+    public static MediaIdentity Episode(
+        Guid itemId,
+        DateTime dateCreated,
+        Guid seriesId,
+        int season,
+        int episode,
+        int? episodeEnd = null,
+        string? seriesTvdb = null,
+        string name = "Andor S01E01")
+        => new(
+            itemId,
+            MediaKind.Episode,
+            dateCreated,
+            MediaKeys.ForEpisode(MediaKeys.EpisodePart(season, episode, episodeEnd)!, seriesTvdb, null, null, seriesId),
+            name);
+}