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/Contracts/LibraryDtos.cs

@@ -0,0 +1,58 @@
+#region license
+// This file is part of Vocaluxe.
+//
+// Vocaluxe is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Vocaluxe is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Vocaluxe. If not, see <http://www.gnu.org/licenses/>.
+#endregion
+
+using System.Collections.Generic;
+
+// Shared between the song server and the game (linked into Vocaluxe.csproj). Keep it free of dependencies.
+// Serialized as camelCase JSON. All paths are relative to the library root and use '/'.
+namespace VocaluxeSongServer.Contracts
+{
+    /// <summary>GET /api/v1/library</summary>
+    public sealed class LibraryDto
+    {
+        /// <summary>Changes whenever any song file is added, removed or changed. Also sent as ETag.</summary>
+        public string Version { get; set; } = "";
+        public string ScannedAt { get; set; } = "";
+        public List<SongDto> Songs { get; set; } = new List<SongDto>();
+        /// <summary>All files in the song folders (text files, audio, video, images), downloadable via /api/v1/files/{path}.</summary>
+        public List<FileDto> Files { get; set; } = new List<FileDto>();
+    }
+
+    public sealed class SongDto
+    {
+        /// <summary>Stable id, derived from the path of the text file.</summary>
+        public string Id { get; set; } = "";
+        /// <summary>Path of the UltraStar text file.</summary>
+        public string Path { get; set; } = "";
+        public string Title { get; set; } = "";
+        public string Artist { get; set; } = "";
+    }
+
+    public sealed class FileDto
+    {
+        public string Path { get; set; } = "";
+        public long Size { get; set; }
+        /// <summary>Last modification, Unix time in milliseconds (UTC).</summary>
+        public long Modified { get; set; }
+    }
+
+    /// <summary>POST /api/v1/files/bundle: returns the files as a zip archive (entries named by their path).</summary>
+    public sealed class BundleRequest
+    {
+        public List<string> Paths { get; set; } = new List<string>();
+    }
+}