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/LedgerStatistics.cs

@@ -0,0 +1,9 @@
+namespace Jellyfin.Plugin.RecentlyAddedGuard.Ledger;
+
+/// <summary>
+/// Counters describing the ledger.
+/// </summary>
+/// <param name="Entries">All entries.</param>
+/// <param name="LiveEntries">Entries with at least one live item.</param>
+/// <param name="RemovedEntries">Entries whose items are gone (kept for the grace period).</param>
+public sealed record LedgerStatistics(int Entries, int LiveEntries, int RemovedEntries);