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/ReconcileResult.cs

@@ -0,0 +1,36 @@
+using System;
+
+namespace Jellyfin.Plugin.RecentlyAddedGuard.Core;
+
+/// <summary>
+/// What the <see cref="Reconciler"/> concluded for an item.
+/// </summary>
+public enum ReconcileAction
+{
+    /// <summary>The item has no identity keys (yet) and was skipped.</summary>
+    Ignored,
+
+    /// <summary>A new medium; a ledger entry was created with the item's date.</summary>
+    Registered,
+
+    /// <summary>A known medium whose date matches the ledger.</summary>
+    Unchanged,
+
+    /// <summary>The item is older than the ledger (e.g. a date from an NFO); the ledger took the earlier date.</summary>
+    FirstAddedLowered,
+
+    /// <summary>The medium returned after the grace period; it counts as new and the ledger took its date.</summary>
+    ReturnedAfterGrace,
+
+    /// <summary>The item is newer than the ledger: an upgrade whose date has to be restored.</summary>
+    UpgradeDetected
+}
+
+/// <summary>
+/// The outcome of reconciling one item with the ledger.
+/// </summary>
+/// <param name="Action">The conclusion.</param>
+/// <param name="Identity">The item.</param>
+/// <param name="ItemDate">The item's "date added" as evaluated (UTC).</param>
+/// <param name="FirstAdded">The ledger's first "date added" after reconciling (UTC).</param>
+public sealed record ReconcileResult(ReconcileAction Action, MediaIdentity Identity, DateTime ItemDate, DateTime FirstAdded);