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/Services/LibraryItemDateWriter.cs

@@ -0,0 +1,59 @@
+using System;
+using System.Collections.Concurrent;
+using System.Threading;
+using System.Threading.Tasks;
+using Jellyfin.Plugin.RecentlyAddedGuard.Core;
+using MediaBrowser.Controller.Library;
+
+namespace Jellyfin.Plugin.RecentlyAddedGuard.Services;
+
+/// <summary>
+/// Writes "date added" through the library manager and remembers which items it is currently saving,
+/// so the resulting <c>ItemUpdated</c> events can be ignored.
+/// </summary>
+public sealed class LibraryItemDateWriter : IItemDateWriter
+{
+    private readonly ILibraryManager _libraryManager;
+    private readonly ConcurrentDictionary<Guid, byte> _inFlight = new();
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="LibraryItemDateWriter"/> class.
+    /// </summary>
+    /// <param name="libraryManager">The library manager.</param>
+    public LibraryItemDateWriter(ILibraryManager libraryManager)
+    {
+        _libraryManager = libraryManager;
+    }
+
+    /// <summary>
+    /// Checks whether the item is being saved by this writer right now.
+    /// </summary>
+    /// <param name="itemId">The item.</param>
+    /// <returns><c>true</c> while a correction of the item is in progress.</returns>
+    public bool IsWriting(Guid itemId) => _inFlight.ContainsKey(itemId);
+
+    /// <inheritdoc />
+    public async Task<bool> TrySetDateCreatedAsync(Guid itemId, DateTime dateCreated, CancellationToken cancellationToken)
+    {
+        var item = _libraryManager.GetItemById(itemId);
+        if (item is null)
+        {
+            return false;
+        }
+
+        _inFlight.TryAdd(itemId, 0);
+        try
+        {
+            item.DateCreated = dateCreated;
+
+            // MetadataEdit also lets the NFO saver (if enabled) write the restored <dateadded>.
+            await _libraryManager.UpdateItemAsync(item, item.GetParent(), ItemUpdateType.MetadataEdit, cancellationToken)
+                .ConfigureAwait(false);
+            return true;
+        }
+        finally
+        {
+            _inFlight.TryRemove(itemId, out _);
+        }
+    }
+}