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/ISongSource.cs

@@ -0,0 +1,73 @@
+using System.Globalization;
+using System.Text;
+using VocaluxeSongServer.Contracts;
+
+namespace VocaluxeSongServer.Sources
+{
+    /// <summary>A place where new songs can be found and downloaded (song database, song repository, ...).</summary>
+    public interface ISongSource
+    {
+        string Id { get; }
+        string Name { get; }
+        string Description { get; }
+        bool Enabled { get; }
+        /// <summary>Why the source is disabled, or empty.</summary>
+        string Hint { get; }
+
+        Task<List<SearchResultDto>> SearchAsync(string query, CancellationToken ct);
+
+        /// <summary>
+        ///     Downloads the song into <paramref name="folder" /> (created by the caller, empty). Must leave a playable
+        ///     UltraStar song there (text file + audio) or throw.
+        /// </summary>
+        Task DownloadAsync(string sourceId, string folder, IProgress<double> progress, CancellationToken ct);
+    }
+
+    public static class CSourceHelper
+    {
+        /// <summary>Lower case, without diacritics and punctuation, single spaces: for matching artists and titles.</summary>
+        public static string Normalize(string s)
+        {
+            var sb = new StringBuilder();
+            foreach (var c in s.Normalize(NormalizationForm.FormD))
+            {
+                var cat = CharUnicodeInfo.GetUnicodeCategory(c);
+                if (cat == UnicodeCategory.NonSpacingMark)
+                {
+                    continue;
+                }
+
+                sb.Append(char.IsLetterOrDigit(c) ? char.ToLowerInvariant(c) : ' ');
+            }
+
+            return string.Join(' ', sb.ToString().Split(' ', StringSplitOptions.RemoveEmptyEntries));
+        }
+
+        /// <summary>True if every word of the query occurs in the text (both normalized).</summary>
+        public static bool Matches(string normalizedQuery, string text)
+        {
+            var normalized = " " + Normalize(text) + " ";
+            return normalizedQuery.Split(' ', StringSplitOptions.RemoveEmptyEntries).All(w => normalized.Contains(w));
+        }
+
+        /// <summary>A file or folder name without characters that are invalid on Windows or Linux.</summary>
+        public static string SafeName(string name)
+        {
+            var invalid = "<>:\"/\\|?*".ToCharArray();
+            var cleaned = new string(name.Select(c => invalid.Contains(c) || char.IsControl(c) ? '_' : c).ToArray()).Trim().TrimEnd('.');
+            return cleaned.Length > 120 ? cleaned[..120] : cleaned;
+        }
+
+        /// <summary>Copies a stream and reports the bytes copied.</summary>
+        public static async Task CopyAsync(Stream source, Stream target, Action<long> onBytes, CancellationToken ct)
+        {
+            var buffer = new byte[81920];
+            int read;
+            while ((read = await source.ReadAsync(buffer, ct)) > 0)
+            {
+                await target.WriteAsync(buffer.AsMemory(0, read), ct);
+                onBytes(read);
+            }
+        }
+    }
+}