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/Plugin.cs

@@ -0,0 +1,65 @@
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using Jellyfin.Plugin.RecentlyAddedGuard.Configuration;
+using MediaBrowser.Common.Configuration;
+using MediaBrowser.Common.Plugins;
+using MediaBrowser.Model.Plugins;
+using MediaBrowser.Model.Serialization;
+
+namespace Jellyfin.Plugin.RecentlyAddedGuard;
+
+/// <summary>
+/// Keeps quality upgrades from Radarr and Sonarr out of "Recently Added".
+/// </summary>
+public class Plugin : BasePlugin<PluginConfiguration>, IHasWebPages
+{
+    /// <summary>
+    /// The permanent plugin id. Never change it: it identifies the plugin in the catalog and in installations.
+    /// </summary>
+    public const string PluginGuid = "6fdfedef-adde-4ac3-a873-62a057c95574";
+
+    private static readonly Guid _pluginId = Guid.Parse(PluginGuid);
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="Plugin"/> class.
+    /// </summary>
+    /// <param name="applicationPaths">The application paths.</param>
+    /// <param name="xmlSerializer">The XML serializer.</param>
+    public Plugin(IApplicationPaths applicationPaths, IXmlSerializer xmlSerializer)
+        : base(applicationPaths, xmlSerializer)
+    {
+        Instance = this;
+    }
+
+    /// <summary>
+    /// Gets the plugin instance; set once Jellyfin has created the plugin.
+    /// </summary>
+    public static Plugin? Instance { get; private set; }
+
+    /// <inheritdoc />
+    public override string Name => "Recently Added Guard";
+
+    /// <inheritdoc />
+    public override Guid Id => _pluginId;
+
+    /// <inheritdoc />
+    public override string Description
+        => "Hält Qualitäts-Upgrades von Radarr und Sonarr aus „Zuletzt hinzugefügt“ fern, indem es das erste Hinzufügedatum pro Film und Episode bewahrt.";
+
+    /// <summary>
+    /// Gets the current configuration, or the defaults before the plugin is loaded.
+    /// </summary>
+    internal static PluginConfiguration CurrentConfiguration => Instance?.Configuration ?? new PluginConfiguration();
+
+    /// <inheritdoc />
+    public IEnumerable<PluginPageInfo> GetPages()
+    {
+        yield return new PluginPageInfo
+        {
+            Name = "RecentlyAddedGuard",
+            DisplayName = Name,
+            EmbeddedResourcePath = string.Format(CultureInfo.InvariantCulture, "{0}.Configuration.configPage.html", GetType().Namespace)
+        };
+    }
+}