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.slnx

@@ -0,0 +1,4 @@
+<Solution>
+  <Project Path="Jellyfin.Plugin.RecentlyAddedGuard/Jellyfin.Plugin.RecentlyAddedGuard.csproj" />
+  <Project Path="Jellyfin.Plugin.RecentlyAddedGuard.Tests/Jellyfin.Plugin.RecentlyAddedGuard.Tests.csproj" />
+</Solution>