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/Library/CLibraryIndex.cs

@@ -8,6 +8,8 @@ namespace VocaluxeSongServer.Library
         public string Path { get; set; } = "/songs";
         /// <summary>Minutes between automatic rescans. 0 = only at startup and via POST /api/v1/library/rescan.</summary>
         public int ScanIntervalMinutes { get; set; } = 10;
+        /// <summary>Folder below the library root for downloaded songs (one subfolder per source).</summary>
+        public string DownloadFolder { get; set; } = "Downloads";
         /// <summary>If set, every /api request needs the header X-Api-Key with this value.</summary>
         public string ApiKey { get; set; } = "";
     }