Constant AVATAR_REF_CLAUSE 

pub(crate) const AVATAR_REF_CLAUSE: &str = "(EXISTS (SELECT 1 FROM users u WHERE u.avatar_url = '/uploads/' || a.path) \
     OR EXISTS (SELECT 1 FROM friend_links f WHERE f.avatar_url = '/uploads/' || a.path))";

直接头像引用判定，供单删接口区分评论引用。