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/Tests/Musify.Application.Tests/YouTube/SearchYouTubeTracksQueryHandlerTests.cs

@@ -1,37 +0,0 @@
-using ErrorOr;
-using Musify.Application.Contracts;
-using Musify.Application.YouTube;
-using NSubstitute;
-using Xunit;
-
-namespace Musify.Application.Tests.YouTube
-{
-    public sealed class SearchYouTubeTracksQueryHandlerTests
-    {
-        private readonly IYouTubeMusicService youTubeMusicService = Substitute.For<IYouTubeMusicService>();
-
-        private SearchYouTubeTracksQueryHandler CreateHandler() => new(youTubeMusicService);
-
-        [Fact]
-        public async Task Handle_EmptyQuery_ReturnsValidationErrorWithoutCallingTheService()
-        {
-            var result = await CreateHandler().Handle(new SearchYouTubeTracksQuery(string.Empty, string.Empty), TestContext.Current.CancellationToken);
-
-            Assert.Equal(ErrorType.Validation, result.FirstError.Type);
-            await youTubeMusicService.DidNotReceiveWithAnyArgs()
-                .SearchSongsAsync(default!, default!, TestContext.Current.CancellationToken);
-        }
-
-        [Fact]
-        public async Task Handle_NonEmptyQuery_DelegatesToTheService()
-        {
-            var expected = new YouTubeSearchResult([], "next-token");
-            youTubeMusicService.SearchSongsAsync("query", "token", Arg.Any<CancellationToken>()).Returns(expected);
-
-            var result = await CreateHandler().Handle(new SearchYouTubeTracksQuery("query", "token"), TestContext.Current.CancellationToken);
-
-            Assert.False(result.IsError);
-            Assert.Same(expected, result.Value);
-        }
-    }
-}