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/Tasks/SyncLedgerTask.cs

@@ -0,0 +1,59 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+using Jellyfin.Plugin.RecentlyAddedGuard.Services;
+using MediaBrowser.Model.Tasks;
+
+namespace Jellyfin.Plugin.RecentlyAddedGuard.Tasks;
+
+/// <summary>
+/// Scheduled task "Ledger synchronisieren": registers unknown items, updates live items and prunes the ledger.
+/// It never changes library items.
+/// </summary>
+public class SyncLedgerTask : IScheduledTask
+{
+    /// <summary>
+    /// The task key.
+    /// </summary>
+    public const string TaskKey = "RecentlyAddedGuardSyncLedger";
+
+    private readonly LedgerSynchronizer _synchronizer;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="SyncLedgerTask"/> class.
+    /// </summary>
+    /// <param name="synchronizer">The synchronizer.</param>
+    public SyncLedgerTask(LedgerSynchronizer synchronizer)
+    {
+        _synchronizer = synchronizer;
+    }
+
+    /// <inheritdoc />
+    public string Name => "Ledger synchronisieren";
+
+    /// <inheritdoc />
+    public string Key => TaskKey;
+
+    /// <inheritdoc />
+    public string Description
+        => "Gleicht das Ledger von Recently Added Guard mit der Bibliothek ab: registriert unbekannte Filme und Episoden, "
+            + "trägt gelöschte Items aus und bereinigt abgelaufene Einträge. Ändert keine Items.";
+
+    /// <inheritdoc />
+    public string Category => "Recently Added Guard";
+
+    /// <inheritdoc />
+    public Task ExecuteAsync(IProgress<double> progress, CancellationToken cancellationToken)
+        => _synchronizer.RunAsync(progress, cancellationToken);
+
+    /// <inheritdoc />
+    public IEnumerable<TaskTriggerInfo> GetDefaultTriggers()
+    {
+        yield return new TaskTriggerInfo
+        {
+            Type = TaskTriggerInfoType.DailyTrigger,
+            TimeOfDayTicks = TimeSpan.FromHours(3).Ticks
+        };
+    }
+}