Generalize YouTube-backed endpoints behind a source parameter

Removes the last YouTube-specific routes (/youtube/*) and merges them
into the general track/album/playlist endpoints, keyed by a `source`
string validated against TrackSource. This mirrors the earlier track
search merge for albums and playlist-track additions:

- GetAlbums now merges local albums with live YouTube Music search
  results, paginated via a youtubeContinuationToken, deduped against
  already-materialized ExternalAlbum rows.
- New GET /albums/external/{source}/{externalId} and
  GET /tracks/external/{source}/{externalId}/stream replace the
  YouTube-only equivalents.
- POST /playlists/{id}/tracks replaces the separate
  .../youtube-tracks endpoint, accepting an optional Source/ExternalId
  pair alongside TrackId.
- AlbumSearchItemResponse/AlbumsSearchResponse follow the same
  discriminated-union + trailing-HasNextPage-field pattern already
  used for track search, so HasNextPage stays a required field in the
  generated OpenAPI schema.
- Frontend routes renamed accordingly (/albums/youtube/[albumId] ->
  /albums/external/[source]/[externalId], etc.) with a single
  /api/thumbnail proxy replacing /api/youtube/thumbnail.

Musify.Api.Endpoints.YouTubeEndpoints and the standalone
SearchYouTubeAlbums query are removed as a result.

backend/Hosts/Musify.Api/Endpoints/AlbumEndpoints.cs

@@ -7,6 +7,8 @@ using Musify.Application.Albums;
 using Musify.Application.Albums.Responses;
 using Musify.Application.Shared;
 using Musify.Application.Tracks.Responses;
+using Musify.Application.YouTube;
+using Musify.Domain.ValueObjects;
 
 namespace Musify.Api.Endpoints;
 
@@ -19,8 +21,8 @@ public static class AlbumEndpoints
 
         group.MapGet("/", GetAlbums)
             .WithName("GetAlbums")
-            .WithSummary("Get Paginated Albums.")
-            .Produces<PaginatedResponse<AlbumApplicationResponse>>();
+            .WithSummary("Get Paginated Albums, Combined With Live YouTube Music Results When A Title Filter Is Given.")
+            .Produces<AlbumsSearchResponse>();
 
         group.MapGet("/{id}", GetAlbumById)
             .WithName("GetAlbumById")
@@ -28,6 +30,12 @@ public static class AlbumEndpoints
             .Produces<AlbumApplicationResponse>()
             .Produces(StatusCodes.Status404NotFound);
 
+        group.MapGet("/external/{source}/{externalId}", GetExternalAlbum)
+            .WithName("GetExternalAlbum")
+            .WithSummary("Get An Album From An External Source (Currently Only YouTube Music) With Its Tracklist.")
+            .Produces<YouTubeAlbumDetail>()
+            .Produces(StatusCodes.Status404NotFound);
+
         group.MapGet("/users/{userId}", GetAlbumsByUserId)
             .WithName("GetAlbumsByUserId")
             .WithSummary("Get Paginated Albums For A User.")
@@ -100,9 +108,10 @@ public static class AlbumEndpoints
         CancellationToken cancellationToken,
         string? title,
         int pageNumber = 1,
-        int pageSize = 10)
+        int pageSize = 10,
+        string? youtubeContinuationToken = null)
     {
-        var result = await mediator.Send(new GetAlbumsQuery(title, pageNumber, pageSize), cancellationToken);
+        var result = await mediator.Send(new GetAlbumsQuery(title, pageNumber, pageSize, youtubeContinuationToken), cancellationToken);
         return result.ToHttpResult();
     }
 
@@ -115,6 +124,19 @@ public static class AlbumEndpoints
         return result.ToHttpResult();
     }
 
+    private static async Task<IResult> GetExternalAlbum(
+        IMediator mediator,
+        string source,
+        string externalId,
+        CancellationToken cancellationToken)
+    {
+        if (!Enum.TryParse<TrackSource>(source, ignoreCase: true, out var trackSource) || trackSource != TrackSource.YouTube)
+            return Results.NotFound($"Unknown or unsupported source '{source}'.");
+
+        var result = await mediator.Send(new GetYouTubeAlbumQuery(externalId), cancellationToken);
+        return result.ToHttpResult();
+    }
+
     private static async Task<IResult> GetAlbumsByUserId(
         IMediator mediator,
         long userId,