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/Contracts/LibraryDtos.cs

@@ -55,4 +55,62 @@ namespace VocaluxeSongServer.Contracts
     {
         public List<string> Paths { get; set; } = new List<string>();
     }
+
+    /// <summary>GET /api/v1/sources: where new songs can be found.</summary>
+    public sealed class SourceDto
+    {
+        public string Id { get; set; } = "";
+        public string Name { get; set; } = "";
+        public string Description { get; set; } = "";
+        public bool Enabled { get; set; }
+        /// <summary>Why the source is disabled (e.g. missing account), or empty.</summary>
+        public string Hint { get; set; } = "";
+    }
+
+    /// <summary>GET /api/v1/search?q=...: results of all enabled sources.</summary>
+    public sealed class SearchResponse
+    {
+        public List<SearchResultDto> Results { get; set; } = new List<SearchResultDto>();
+        /// <summary>Sources that failed for this search: source id -> error message.</summary>
+        public Dictionary<string, string> Errors { get; set; } = new Dictionary<string, string>();
+    }
+
+    public sealed class SearchResultDto
+    {
+        public string Source { get; set; } = "";
+        /// <summary>Id of the song within its source; used to download it.</summary>
+        public string SourceId { get; set; } = "";
+        public string Artist { get; set; } = "";
+        public string Title { get; set; } = "";
+        /// <summary>Extra info shown with the result (language, year, license, ...).</summary>
+        public string Details { get; set; } = "";
+        public string CoverUrl { get; set; } = "";
+        /// <summary>True if a song with the same artist and title is already in the library.</summary>
+        public bool InLibrary { get; set; }
+    }
+
+    /// <summary>POST /api/v1/downloads</summary>
+    public sealed class DownloadRequest
+    {
+        public string Source { get; set; } = "";
+        public string SourceId { get; set; } = "";
+        public string Artist { get; set; } = "";
+        public string Title { get; set; } = "";
+    }
+
+    /// <summary>GET /api/v1/downloads: queued, running and finished downloads (newest first).</summary>
+    public sealed class DownloadJobDto
+    {
+        public string Id { get; set; } = "";
+        public string Source { get; set; } = "";
+        public string SourceId { get; set; } = "";
+        public string Artist { get; set; } = "";
+        public string Title { get; set; } = "";
+        /// <summary>queued, running, done or failed</summary>
+        public string State { get; set; } = "";
+        /// <summary>0..1</summary>
+        public double Progress { get; set; }
+        public string Message { get; set; } = "";
+        public string Created { get; set; } = "";
+    }
 }