Adds POST /admin/posts/feature, POST /admin/posts/unfeature
(both auth-required, admin-checked inside controller via the
use case's ForbiddenException), and public GET /posts/featured.
Post serialization now includes featureSlot.
Wires PostController (recent, show, create, delete, listByUser)
and CommentController (listForPost, create, delete) to the
existing use cases. Posts and comments expose author display
names alongside user IDs. CommentRepository binding added to
RepositoryServiceProvider.
