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/Hosts/Musify.Api/Endpoints/TrackEndpoints.cs

@@ -19,8 +19,8 @@ public static class TrackEndpoints
 
         group.MapGet("/", GetTracks)
             .WithName("GetTracks")
-            .WithSummary("Get Paginated Tracks.")
-            .Produces<PaginatedResponse<TrackApplicationResponse>>();
+            .WithSummary("Get Paginated Tracks, Combined With Live YouTube Music Results When A Name Filter Is Given.")
+            .Produces<TracksSearchResponse>();
 
         group.MapGet("/{id}", GetTrackById)
             .WithName("GetTrackById")
@@ -86,9 +86,10 @@ public static class TrackEndpoints
         CancellationToken cancellationToken,
         string? name,
         int pageNumber = 1,
-        int pageSize = 10)
+        int pageSize = 10,
+        string? youtubeContinuationToken = null)
     {
-        var result = await mediator.Send(new GetTracksQuery(name, pageNumber, pageSize), cancellationToken);
+        var result = await mediator.Send(new GetTracksQuery(name, pageNumber, pageSize, youtubeContinuationToken), cancellationToken);
         return result.ToHttpResult();
     }