feat(api): expose the album endpoints

Eight routes under /albums covering the listing, the detail, the tracklist and
the owner-only writes, wired to the same validation filter and ErrorOr mapping
the playlists use.

backend/Musify.Api/Program.cs

@@ -38,6 +38,7 @@ app.UseAuthentication();
 app.UseAuthorization();
 
 app
+    .MapAlbumEndpoints()
     .MapPlayListEndpoints()
     .MapTrackEndpoints()
     .MapUserEndpoints()