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>

VocaluxeSongServer/Dockerfile

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+# Build from the repository root:  docker build -f VocaluxeSongServer/Dockerfile -t vocaluxe-song-server .
+FROM mcr.microsoft.com/dotnet/sdk:10.0 AS build
+WORKDIR /src
+COPY Directory.Build.props NuGet.Config ./
+COPY VocaluxeSongServer/VocaluxeSongServer.csproj VocaluxeSongServer/
+RUN dotnet restore VocaluxeSongServer/VocaluxeSongServer.csproj
+COPY VocaluxeSongServer/ VocaluxeSongServer/
+RUN dotnet publish VocaluxeSongServer/VocaluxeSongServer.csproj -c ReleaseLinux --no-restore -o /app
+
+FROM mcr.microsoft.com/dotnet/aspnet:10.0
+WORKDIR /app
+COPY --from=build /app .
+# The song library (read-only is enough); see README.md
+VOLUME /songs
+ENV Library__Path=/songs
+EXPOSE 8080
+USER $APP_UID
+ENTRYPOINT ["dotnet", "VocaluxeSongServer.dll"]