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>

VocaluxeLib/Songs/CSong.cs

@@ -338,6 +338,41 @@ namespace VocaluxeLib.Songs
             return Path.Combine(Folder, Video);
         }
 
+        public string GetCover()
+        {
+            return Cover == "" ? "" : Path.Combine(Folder, Cover);
+        }
+
+        /// <summary>
+        ///     Full paths of the files needed to sing the song: audio (or instrumental/vocals), video, backgrounds and cover.
+        /// </summary>
+        public List<string> GetMediaFiles(bool includeVideo = true)
+        {
+            var files = new List<string> { GetMP3() };
+            if (HasInstrumental())
+            {
+                files.Add(GetInstrumental());
+            }
+
+            if (HasVocals())
+            {
+                files.Add(GetVocals());
+            }
+
+            if (includeVideo && !string.IsNullOrEmpty(Video))
+            {
+                files.Add(GetVideo());
+            }
+
+            files.AddRange(BackgroundFileNames.Where(bg => !string.IsNullOrEmpty(bg)).Select(bg => Path.Combine(Folder, bg)));
+            if (Cover != "")
+            {
+                files.Add(GetCover());
+            }
+
+            return files;
+        }
+
         public void LoadSmallCover()
         {
             if (_CoverTextureSmall != null)
@@ -347,7 +382,9 @@ namespace VocaluxeLib.Songs
 
             if (Cover != "")
             {
-                if (CBase.DataBase.GetCover(Path.Combine(Folder, Cover), ref _CoverTextureSmall, CBase.Config.GetCoverSize()))
+                // Songs from a song server: download the cover first (cover loading runs on background threads).
+                CSongFiles.EnsureLocal(GetCover());
+                if (CBase.DataBase.GetCover(GetCover(), ref _CoverTextureSmall, CBase.Config.GetCoverSize()))
                 {
                     return;
                 }