publishPost method
Implementation
Future<void> publishPost(String postId) async {
final doc = await _firestore.collection('posts').doc(postId).get();
final post = PostModel.fromFirestore(doc);
final published = post.copyWith(
status: PostStatus.published,
publishedAt: DateTime.now(),
);
await _firestore.collection('posts').doc(postId).update(
published.toFirestoreUpdate(),
);
}