ALTER TABLE servers
  ADD COLUMN agent_key VARCHAR(64) DEFAULT NULL AFTER ssh_key,
  ADD COLUMN agent_installed TINYINT(1) NOT NULL DEFAULT 0 AFTER agent_key,
  ADD COLUMN agent_install_path VARCHAR(255) DEFAULT NULL AFTER agent_installed,
  ADD COLUMN agent_last_seen_at DATETIME DEFAULT NULL AFTER agent_install_path;

CREATE UNIQUE INDEX idx_servers_agent_key ON servers(agent_key);

UPDATE servers SET agent_key = LOWER(HEX(RANDOM_BYTES(32))) WHERE agent_key IS NULL;