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/Core/IItemDateWriter.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Jellyfin.Plugin.RecentlyAddedGuard.Core;
+
+/// <summary>
+/// Writes the "date added" of a library item.
+/// </summary>
+public interface IItemDateWriter
+{
+    /// <summary>
+    /// Sets the item's "date added" and saves it.
+    /// </summary>
+    /// <param name="itemId">The item.</param>
+    /// <param name="dateCreated">The new "date added" (UTC).</param>
+    /// <param name="cancellationToken">The cancellation token.</param>
+    /// <returns><c>false</c> if the item no longer exists.</returns>
+    Task<bool> TrySetDateCreatedAsync(Guid itemId, DateTime dateCreated, CancellationToken cancellationToken);
+}