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>

Directory.Build.props

@@ -0,0 +1,20 @@
+<Project>
+  <!-- Shared settings for the plugin and the test project. -->
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <LangVersion>latest</LangVersion>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>disable</ImplicitUsings>
+    <!-- Overwritten by the release workflow (jprm) with the version from the tag. -->
+    <Version>1.0.0.0</Version>
+    <Authors>menzeljonas</Authors>
+    <Deterministic>true</Deterministic>
+    <AnalysisLevel>latest-recommended</AnalysisLevel>
+    <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
+    <GenerateDocumentationFile>true</GenerateDocumentationFile>
+  </PropertyGroup>
+
+  <PropertyGroup Condition=" '$(Configuration)' == 'Release' ">
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+  </PropertyGroup>
+</Project>