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/Core/Musify.Application/Tracks/GetTracks.cs

@@ -67,7 +67,7 @@ namespace Musify.Application.Tracks
                 (youtubeItems, nextContinuationToken, youtubeUnavailable) =
                     await SearchYouTubeAsync(request.Name, request.YoutubeContinuationToken, knownVideoIds, cancellationToken);
 
-            return new TracksSearchResponse(
+            return TracksSearchResponse.Create(
                 [.. localItems, .. youtubeItems],
                 pagedEntities.PageNumber,
                 pagedEntities.PageSize,

backend/Core/Musify.Application/Tracks/Responses/TracksSearchResponse.cs

@@ -15,8 +15,24 @@ namespace Musify.Application.Tracks.Responses
         bool HasPreviousPage,
         bool HasNextLocalPage,
         string? NextYoutubeContinuationToken,
-        bool YoutubeUnavailable)
+        bool YoutubeUnavailable,
+        bool HasNextPage)
     {
-        public bool HasNextPage => HasNextLocalPage || !string.IsNullOrEmpty(NextYoutubeContinuationToken);
+        // HasNextPage is a real field (not a computed property) so it shows up as a required member
+        // of the generated OpenAPI schema, same as every other field here — a get-only property added
+        // outside the primary constructor gets reflected as optional instead, which is never true here.
+        public static TracksSearchResponse Create(
+            IReadOnlyList<TrackSearchItemResponse> items,
+            int pageNumber,
+            int pageSize,
+            int pageCount,
+            int totalItemCount,
+            bool hasPreviousPage,
+            bool hasNextLocalPage,
+            string? nextYoutubeContinuationToken,
+            bool youtubeUnavailable) =>
+            new(items, pageNumber, pageSize, pageCount, totalItemCount, hasPreviousPage, hasNextLocalPage,
+                nextYoutubeContinuationToken, youtubeUnavailable,
+                HasNextPage: hasNextLocalPage || !string.IsNullOrEmpty(nextYoutubeContinuationToken));
     }
 }