Song server: search and download songs, mobile web UI

- Sources: "free" (Creative Commons songs with media from
  github.com/UltraStar-Deluxe/songs) and "usdb" (usdb.animux.de with an
  account; audio/video via yt-dlp from the song's #VIDEO hints or YouTube
  links in the comments, covers from fanart.tv/YouTube)
- Aggregated search (all sources in parallel, in-library flag), download
  queue into <library>/Downloads/<source>/<Artist - Title>, then rescan
- Web UI in wwwroot (plain JS, no build): search, downloads with progress,
  library; phone first, dark mode, installable
- Docker image with yt-dlp, ffmpeg and deno; library mounted writable
- Tests for matching, USDB parsing and song rewriting

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

VocaluxeSongServer/Sources/CGitHubSongSource.cs

@@ -0,0 +1,131 @@
+using System.Text.Json;
+using VocaluxeSongServer.Contracts;
+
+namespace VocaluxeSongServer.Sources
+{
+    /// <summary>
+    ///     Openly licensed songs with all media from a GitHub repository, by default UltraStar-Deluxe/songs
+    ///     (Creative Commons / public domain; one song per folder, license.txt in every folder).
+    ///     The repository tree is read with one API call and cached for a day (the API allows 60 calls/hour without token).
+    /// </summary>
+    public sealed class CGitHubSongSource : ISongSource
+    {
+        private sealed record CSong(string Folder, string Artist, string Title, string License, List<(string Path, long Size)> Files);
+
+        private readonly HttpClient _Http;
+        private readonly string _Repo;
+        private readonly string _Branch;
+        private readonly SemaphoreSlim _Lock = new(1, 1);
+        private List<CSong>? _Catalog;
+        private DateTime _CatalogTime;
+
+        public CGitHubSongSource(HttpClient http, string repo = "UltraStar-Deluxe/songs", string branch = "master")
+        {
+            _Http = http;
+            _Repo = repo;
+            _Branch = branch;
+        }
+
+        public string Id => "free";
+        public string Name => "Free songs";
+        public string Description => "Creative Commons and public domain songs with audio, from github.com/" + _Repo;
+        public bool Enabled => true;
+        public string Hint => "";
+
+        private async Task<List<CSong>> _GetCatalogAsync(CancellationToken ct)
+        {
+            await _Lock.WaitAsync(ct);
+            try
+            {
+                if (_Catalog != null && DateTime.UtcNow - _CatalogTime < TimeSpan.FromDays(1))
+                {
+                    return _Catalog;
+                }
+
+                using var request = new HttpRequestMessage(HttpMethod.Get, $"https://api.github.com/repos/{_Repo}/git/trees/{_Branch}?recursive=1");
+                request.Headers.UserAgent.ParseAdd("VocaluxeSongServer");
+                using var response = await _Http.SendAsync(request, ct);
+                response.EnsureSuccessStatusCode();
+                using var doc = JsonDocument.Parse(await response.Content.ReadAsStringAsync(ct));
+
+                var files = doc.RootElement.GetProperty("tree").EnumerateArray()
+                               .Where(e => e.GetProperty("type").GetString() == "blob")
+                               .Select(e => (Path: e.GetProperty("path").GetString()!, Size: e.TryGetProperty("size", out var s) ? s.GetInt64() : 0))
+                               .ToList();
+
+                var songs = new List<CSong>();
+                foreach (var folder in files.GroupBy(f => f.Path.Contains('/') ? f.Path[..f.Path.LastIndexOf('/')] : ""))
+                {
+                    var hasSong = folder.Any(f => f.Path.EndsWith(".txt", StringComparison.OrdinalIgnoreCase) &&
+                                                  !f.Path.EndsWith("/license.txt", StringComparison.OrdinalIgnoreCase));
+                    if (!hasSong || folder.Key == "")
+                    {
+                        continue;
+                    }
+
+                    // Folder names are "Artist - Title"; the category is the parent folder ("Creative Commons", ...).
+                    var name = folder.Key[(folder.Key.LastIndexOf('/') + 1)..];
+                    var category = folder.Key.Contains('/') ? folder.Key[..folder.Key.LastIndexOf('/')] : "";
+                    var dash = name.IndexOf(" - ", StringComparison.Ordinal);
+                    var artist = dash > 0 ? name[..dash] : "";
+                    var title = dash > 0 ? name[(dash + 3)..] : name;
+                    songs.Add(new CSong(folder.Key, artist, title, category, folder.ToList()));
+                }
+
+                _Catalog = songs;
+                _CatalogTime = DateTime.UtcNow;
+                return songs;
+            }
+            finally
+            {
+                _Lock.Release();
+            }
+        }
+
+        private string _RawUrl(string path)
+        {
+            return $"https://raw.githubusercontent.com/{_Repo}/{_Branch}/" + string.Join('/', path.Split('/').Select(Uri.EscapeDataString));
+        }
+
+        public async Task<List<SearchResultDto>> SearchAsync(string query, CancellationToken ct)
+        {
+            var normalized = CSourceHelper.Normalize(query);
+            var catalog = await _GetCatalogAsync(ct);
+            return catalog.Where(s => CSourceHelper.Matches(normalized, s.Artist + " " + s.Title))
+                          .Select(s =>
+                          {
+                              var cover = s.Files.FirstOrDefault(f => Path.GetFileNameWithoutExtension(f.Path).Equals("cover", StringComparison.OrdinalIgnoreCase)).Path;
+                              return new SearchResultDto
+                              {
+                                  Source = Id,
+                                  SourceId = s.Folder,
+                                  Artist = s.Artist,
+                                  Title = s.Title,
+                                  Details = $"{s.License} · {s.Files.Sum(f => f.Size) / 1_000_000.0:0.#} MB",
+                                  CoverUrl = cover != null ? _RawUrl(cover) : ""
+                              };
+                          })
+                          .ToList();
+        }
+
+        public async Task DownloadAsync(string sourceId, string folder, IProgress<double> progress, CancellationToken ct)
+        {
+            var song = (await _GetCatalogAsync(ct)).FirstOrDefault(s => s.Folder == sourceId) ?? throw new InvalidOperationException("Song not found in " + _Repo);
+            var total = Math.Max(1, song.Files.Sum(f => f.Size));
+            long done = 0;
+            foreach (var file in song.Files)
+            {
+                var target = Path.Combine(folder, CSourceHelper.SafeName(Path.GetFileName(file.Path)));
+                using var response = await _Http.GetAsync(_RawUrl(file.Path), HttpCompletionOption.ResponseHeadersRead, ct);
+                response.EnsureSuccessStatusCode();
+                await using var source = await response.Content.ReadAsStreamAsync(ct);
+                await using var stream = File.Create(target);
+                await CSourceHelper.CopyAsync(source, stream, n =>
+                {
+                    done += n;
+                    progress.Report((double)done / total);
+                }, ct);
+            }
+        }
+    }
+}