Combine track search with live YouTube Music results

GET /tracks now merges the local catalog page with a live YouTube Music
search (deduplicated against tracks already provisioned locally) whenever a
name filter is given, appending the YouTube hits after the local matches and
carrying a continuation token for further pages. Each item is tagged with its
source via the new TrackSearchItemResponse/TracksSearchResponse shapes,
wrapping the existing TrackApplicationResponse or a raw YouTubeSongResult so
neither type had to change.

GET /youtube/search is removed along with its now-unused handler; the
web-player's explore page, its /api/tracks proxy, and the YouTube filler
helper are updated to call the combined endpoint instead of hitting the two
separately.

backend/Core/Musify.Application/YouTube/SearchYouTubeTracks.cs

@@ -1,21 +0,0 @@
-using ErrorOr;
-using Mediator;
-using Musify.Application.Contracts;
-
-namespace Musify.Application.YouTube
-{
-    public record SearchYouTubeTracksQuery(string Query, string ContinuationToken)
-        : IQuery<ErrorOr<YouTubeSearchResult>>;
-
-    public class SearchYouTubeTracksQueryHandler(IYouTubeMusicService youTubeMusicService)
-        : IQueryHandler<SearchYouTubeTracksQuery, ErrorOr<YouTubeSearchResult>>
-    {
-        public async ValueTask<ErrorOr<YouTubeSearchResult>> Handle(SearchYouTubeTracksQuery request, CancellationToken cancellationToken)
-        {
-            if (string.IsNullOrEmpty(request.Query))
-                return Error.Validation(description: "The search query must not be empty.");
-
-            return await youTubeMusicService.SearchSongsAsync(request.Query, request.ContinuationToken, cancellationToken);
-        }
-    }
-}