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/Program.cs

@@ -5,6 +5,7 @@ using Microsoft.AspNetCore.StaticFiles;
 using Microsoft.Extensions.Options;
 using VocaluxeSongServer.Contracts;
 using VocaluxeSongServer.Library;
+using VocaluxeSongServer.Sources;
 
 var builder = WebApplication.CreateBuilder(args);
 
@@ -13,6 +14,21 @@ builder.Services.Configure<CLibraryOptions>(builder.Configuration.GetSection("Li
 builder.Services.AddSingleton<CLibraryScanner>();
 builder.Services.AddSingleton<CLibraryIndex>();
 builder.Services.AddHostedService(sp => sp.GetRequiredService<CLibraryIndex>());
+// Song sources for search and download (see Sources/). USDB needs Usdb__User and Usdb__Password.
+builder.Services.AddSingleton<CYtDlp>();
+builder.Services.AddHttpClient("sources", c => c.Timeout = TimeSpan.FromMinutes(10));
+builder.Services.AddSingleton<ISongSource>(sp => new CGitHubSongSource(sp.GetRequiredService<IHttpClientFactory>().CreateClient("sources")));
+builder.Services.AddSingleton<ISongSource>(sp => new CUsdbSongSource(
+    new HttpClient(new HttpClientHandler { CookieContainer = new System.Net.CookieContainer(), AutomaticDecompression = System.Net.DecompressionMethods.All })
+    {
+        Timeout = TimeSpan.FromMinutes(2),
+        DefaultRequestHeaders = { { "User-Agent", "VocaluxeSongServer" } }
+    },
+    builder.Configuration.GetSection("Usdb").Get<CUsdbOptions>() ?? new CUsdbOptions(),
+    sp.GetRequiredService<CYtDlp>(), sp.GetRequiredService<ILogger<CUsdbSongSource>>()));
+builder.Services.AddSingleton<CSongSearch>();
+builder.Services.AddSingleton<CDownloadQueue>();
+builder.Services.AddHostedService(sp => sp.GetRequiredService<CDownloadQueue>());
 builder.Services.AddResponseCompression(o =>
 {
     o.EnableForHttps = true;
@@ -23,6 +39,9 @@ builder.Services.ConfigureHttpJsonOptions(o => o.SerializerOptions.PropertyNamin
 
 var app = builder.Build();
 app.UseResponseCompression();
+// The web UI (wwwroot): search and download songs, browse the library. Mobile first.
+app.UseDefaultFiles();
+app.UseStaticFiles();
 
 app.MapGet("/health", (CLibraryIndex index) => index.Current == null
     ? Results.Json(new { status = "scanning" }, statusCode: 503)
@@ -121,6 +140,28 @@ api.MapPost("/files/bundle", async (BundleRequest request, CLibraryIndex index,
     return Results.File(buffer, "application/zip", "bundle.zip");
 });
 
+// Song search and downloads
+api.MapGet("/sources", (CSongSearch search) => search.Sources.Select(s => new SourceDto
+{
+    Id = s.Id, Name = s.Name, Description = s.Description, Enabled = s.Enabled, Hint = s.Hint
+}));
+
+api.MapGet("/search", (string q, CSongSearch search, CancellationToken ct) => search.SearchAsync(q, ct));
+
+api.MapGet("/downloads", (CDownloadQueue queue) => queue.Jobs);
+
+api.MapPost("/downloads", (DownloadRequest request, CDownloadQueue queue) =>
+{
+    try
+    {
+        return Results.Json(queue.Enqueue(request));
+    }
+    catch (ArgumentException e)
+    {
+        return Results.Problem(title: e.Message, statusCode: StatusCodes.Status400BadRequest);
+    }
+});
+
 app.Run();
 
 public partial class Program { }