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/Ledger/LedgerEntry.cs

@@ -0,0 +1,48 @@
+using System;
+using System.Collections.Generic;
+
+namespace Jellyfin.Plugin.RecentlyAddedGuard.Ledger;
+
+/// <summary>
+/// The first "date added" of one logical medium (a movie or an episode), independent of its files.
+/// </summary>
+/// <remarks>
+/// <see cref="Keys"/> and <see cref="LiveItemIds"/> are indexed by the <see cref="LedgerStore"/>;
+/// change them only through the store so the indexes stay consistent.
+/// </remarks>
+public sealed class LedgerEntry
+{
+    /// <summary>
+    /// Gets or sets all alias keys of the medium (e.g. <c>movie:tmdb:603</c>).
+    /// </summary>
+#pragma warning disable CA1819 // Plain persistence model; the store owns all mutations.
+    public string[] Keys { get; set; } = [];
+#pragma warning restore CA1819
+
+    /// <summary>
+    /// Gets or sets the protected first "date added" (UTC).
+    /// </summary>
+    public DateTime FirstAdded { get; set; }
+
+    /// <summary>
+    /// Gets or sets when an item of this medium was last seen (UTC).
+    /// </summary>
+    public DateTime LastSeen { get; set; }
+
+    /// <summary>
+    /// Gets or sets when the last live item of this medium disappeared (UTC); <c>null</c> while items exist.
+    /// </summary>
+    public DateTime? RemovedAt { get; set; }
+
+    /// <summary>
+    /// Gets or sets the ids of the Jellyfin items currently representing this medium.
+    /// </summary>
+#pragma warning disable CA2227 // Plain persistence model; the store owns all mutations.
+    public HashSet<Guid> LiveItemIds { get; set; } = [];
+#pragma warning restore CA2227
+
+    /// <summary>
+    /// Gets or sets a display name for logs and the configuration page.
+    /// </summary>
+    public string DisplayName { get; set; } = string.Empty;
+}