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/MediaIdentity.cs

@@ -0,0 +1,31 @@
+using System;
+using System.Collections.Generic;
+
+namespace Jellyfin.Plugin.RecentlyAddedGuard.Core;
+
+/// <summary>
+/// Kind of logical medium tracked by the ledger.
+/// </summary>
+public enum MediaKind
+{
+    /// <summary>A movie.</summary>
+    Movie,
+
+    /// <summary>A TV episode (or a multi-episode file).</summary>
+    Episode
+}
+
+/// <summary>
+/// Jellyfin-independent snapshot of a library item, as consumed by the <see cref="Reconciler"/>.
+/// </summary>
+/// <param name="ItemId">The Jellyfin item id.</param>
+/// <param name="Kind">The kind of medium.</param>
+/// <param name="DateCreated">The item's current "date added" (UTC).</param>
+/// <param name="Keys">All identity keys of the logical medium; empty if the item cannot be identified yet.</param>
+/// <param name="DisplayName">Human readable name for logs and the configuration page.</param>
+public sealed record MediaIdentity(
+    Guid ItemId,
+    MediaKind Kind,
+    DateTime DateCreated,
+    IReadOnlyList<string> Keys,
+    string DisplayName);