Song server: central song library with on-demand downloads

- VocaluxeSongServer: ASP.NET Core server that scans an UltraStar library and
  serves the song list (ETag), single files (Range) and zip bundles; optional
  API key; Dockerfile and Gitea workflow for git.disi.dev/projects/vocaluxe-song-server
- Game: config section SongServer (Url, ApiKey). At song loading, the song text
  files are mirrored into a local cache that is loaded like a song folder;
  without connection the last library is used
- VocaluxeLib: CSongFiles hook for all song file access; media are downloaded on
  demand (covers in the background, preview audio, everything before singing
  with progress in the sing screen)
- Tests for scanner, API and downloads (incl. resume); AGENTS.md for the fork

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Tests/Tests.csproj

@@ -15,6 +15,7 @@
     <PackageReference Include="NUnit3TestAdapter" Version="6.3.1" PrivateAssets="all" />
     <PackageReference Include="NunitXml.TestLogger" Version="8.1.0" />
     <PackageReference Include="coverlet.collector" Version="10.1.0" PrivateAssets="all" />
+    <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="10.0.14" />
   </ItemGroup>
   <ItemGroup>
     <Compile Update="VocaluxeLib\Log\CLogTest.cs">
@@ -32,5 +33,7 @@
     <ProjectReference Include="..\VocaluxeLib\VocaluxeLib.csproj" SetPlatform="Platform=AnyCPU" />
     <ProjectReference Include="..\Vocaluxe\Vocaluxe.csproj" />
     <ProjectReference Include="..\VocaluxeWeb\VocaluxeWeb.csproj" SetPlatform="Platform=AnyCPU" />
+    <!-- Alias: the contract types also exist in Vocaluxe (linked source) -->
+    <ProjectReference Include="..\VocaluxeSongServer\VocaluxeSongServer.csproj" SetPlatform="Platform=AnyCPU" Aliases="songserver" />
   </ItemGroup>
 </Project>