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.Tests/Jellyfin.Plugin.RecentlyAddedGuard.Tests.csproj

@@ -0,0 +1,25 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <IsTestProject>true</IsTestProject>
+    <IsPackable>false</IsPackable>
+    <!-- Keeps `dotnet publish` of the solution (jprm) from publishing the tests. -->
+    <IsPublishable>false</IsPublishable>
+    <RootNamespace>Jellyfin.Plugin.RecentlyAddedGuard.Tests</RootNamespace>
+    <!-- Test names use underscores; test classes need no XML docs. -->
+    <NoWarn>$(NoWarn);CA1707;CS1591</NoWarn>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.9.0" />
+    <PackageReference Include="xunit.v3" Version="3.2.2" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.5" PrivateAssets="all" />
+    <PackageReference Include="Microsoft.Extensions.Logging.Abstractions" Version="10.0.11" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Jellyfin.Plugin.RecentlyAddedGuard\Jellyfin.Plugin.RecentlyAddedGuard.csproj" />
+  </ItemGroup>
+
+</Project>