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/Jellyfin.Plugin.RecentlyAddedGuard.csproj

@@ -0,0 +1,24 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <RootNamespace>Jellyfin.Plugin.RecentlyAddedGuard</RootNamespace>
+    <AssemblyName>Jellyfin.Plugin.RecentlyAddedGuard</AssemblyName>
+    <CopyLocalLockFileAssemblies>false</CopyLocalLockFileAssemblies>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <!-- Provided by the Jellyfin server at runtime; only the plugin DLL is shipped. -->
+    <PackageReference Include="Jellyfin.Controller" Version="12.2.0" ExcludeAssets="runtime" />
+    <PackageReference Include="Jellyfin.Model" Version="12.2.0" ExcludeAssets="runtime" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <None Remove="Configuration\configPage.html" />
+    <EmbeddedResource Include="Configuration\configPage.html" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <InternalsVisibleTo Include="Jellyfin.Plugin.RecentlyAddedGuard.Tests" />
+  </ItemGroup>
+
+</Project>