Replace hidden flag with post state.

Store post state as text so Go owns the allowed values and future states such as locked remain representable without a database enum migration.

internal/store/memory.go

@@ -463,7 +463,7 @@ func (m *Memory) ListRootPosts(_ context.Context, postDate, viewerID string) ([]
 	defer m.mu.Unlock()
 	posts := make([]Post, 0)
 	for _, post := range m.posts {
-		if post.ParentID != nil || post.PostDate != postDate || post.Hidden {
+		if post.ParentID != nil || post.PostDate != postDate || post.PostState == PostStateHidden {
 			continue
 		}
 		cp := clonePostWithAuthor(post, m.users)
@@ -496,7 +496,7 @@ func (m *Memory) VotePost(_ context.Context, userID, postID string, value int) e
 		return fmt.Errorf("invalid vote")
 	}
 	post, ok := m.posts[postID]
-	if !ok || post.ParentID != nil || post.Hidden {
+	if !ok || post.ParentID != nil || post.PostState == PostStateHidden {
 		return ErrPostNotVotable
 	}
 	if m.postVotes[postID] == nil {