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/GuardOptions.cs

@@ -0,0 +1,27 @@
+using System;
+
+namespace Jellyfin.Plugin.RecentlyAddedGuard.Core;
+
+/// <summary>
+/// The settings the core logic depends on, decoupled from the Jellyfin plugin configuration.
+/// </summary>
+/// <param name="GraceDays">Days within which a returning medium counts as an upgrade.</param>
+/// <param name="ToleranceSeconds">Date differences up to this many seconds are ignored.</param>
+/// <param name="DryRun">Only log detected upgrades, never change items.</param>
+public sealed record GuardOptions(int GraceDays, int ToleranceSeconds, bool DryRun)
+{
+    /// <summary>
+    /// Gets the defaults (14 days, 60 seconds, no dry run).
+    /// </summary>
+    public static GuardOptions Default { get; } = new(14, 60, false);
+
+    /// <summary>
+    /// Gets the grace period.
+    /// </summary>
+    public TimeSpan GracePeriod => TimeSpan.FromDays(Math.Max(0, GraceDays));
+
+    /// <summary>
+    /// Gets the tolerance for date comparisons.
+    /// </summary>
+    public TimeSpan Tolerance => TimeSpan.FromSeconds(Math.Max(0, ToleranceSeconds));
+}