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/TrackEndpoints.cs

@@ -3,6 +3,9 @@ using Musify.Application.Tracks;
 using Musify.Application.Tracks.Responses;
 using Musify.Application.Shared;
 using Musify.Application.Contracts;
+using Musify.Application.YouTube;
+using Musify.Application.YouTube.Responses;
+using Musify.Domain.ValueObjects;
 using Musify.Api.Authentication;
 using Musify.Api.DataTransferObjects.Tracks;
 using Musify.Api.Extensions;
@@ -41,6 +44,14 @@ public static class TrackEndpoints
             .Produces(StatusCodes.Status401Unauthorized)
             .Produces(StatusCodes.Status404NotFound);
 
+        group.MapGet("/external/{source}/{externalId}/stream", GetExternalTrackStream)
+            .WithName("GetExternalTrackStream")
+            .WithSummary("Resolve Playback For A Track From An External Source (Currently Only YouTube Music): Server Stream If Downloaded, Direct Stream From The Source Otherwise. Works Anonymously When The Playback Configuration Allows It.")
+            .Produces<YouTubeStreamResponse>()
+            .Produces(StatusCodes.Status401Unauthorized)
+            .Produces(StatusCodes.Status404NotFound)
+            .Produces(StatusCodes.Status409Conflict);
+
         group.MapGet("/users/{userId}", GetTracksByUserId)
             .WithName("GetTracksByUserId")
             .WithSummary("Get Paginated Tracks For A User.")
@@ -129,6 +140,20 @@ public static class TrackEndpoints
         return result.ToHttpResult();
     }
 
+    private static async Task<IResult> GetExternalTrackStream(
+        IMediator mediator,
+        CurrentUser currentUser,
+        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 ResolveYouTubeTrackStreamCommand(externalId, currentUser.Id), cancellationToken);
+        return result.ToHttpResult();
+    }
+
     private static async Task<IResult> GetTracksByUserId(
         IMediator mediator,
         long userId,