text: create table subscriptions
  (
      id          bigint unsigned auto_increment        null,
      topic_id    bigint unsigned                       not null,
      callback    varchar(1024)                         not null,
      secret      varchar(200)                          null,
      lease       int       default 0                   null,
      created_at  timestamp default current_timestamp() not null,
      expires_at  timestamp                             null,
      constraint subscriptions_pk
          primary key (id)
  );
  
  create index subscriptions_topic_index
      on subscriptions (topic_id);
  
  create index subscriptions_topic_callback_index
      on subscriptions (topic_id, callback);
  
  create table topics
  (
      id    bigint unsigned auto_increment primary key,
      topic varchar(512) not null
  );
  
  create index topics_topic_index on topics (topic);