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/IExternalDateSource.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Jellyfin.Plugin.RecentlyAddedGuard.Core;
+
+/// <summary>
+/// An external source for the first "date added" of a medium, e.g. the import history of Radarr or Sonarr.
+/// </summary>
+/// <remarks>
+/// Extension point for v2: implementations registered in the DI container are consulted by the
+/// ledger synchronization, and earlier dates lower the ledger via <see cref="Reconciler.ApplyExternalFirstAdded"/>.
+/// v1 ships no implementation.
+/// </remarks>
+public interface IExternalDateSource
+{
+    /// <summary>
+    /// Gets the name of the source for logs.
+    /// </summary>
+    string Name { get; }
+
+    /// <summary>
+    /// Looks up when the medium was first imported.
+    /// </summary>
+    /// <param name="identity">The medium.</param>
+    /// <param name="cancellationToken">The cancellation token.</param>
+    /// <returns>The first import date (UTC), or <c>null</c> if unknown.</returns>
+    Task<DateTime?> GetFirstAddedAsync(MediaIdentity identity, CancellationToken cancellationToken);
+}