track update time in host table

docs/schema.sql

@@ -160,6 +160,7 @@ CREATE TABLE host (
 	id SERIAL NOT NULL PRIMARY KEY,
 	user_id INTEGER NOT NULL REFERENCES users (id),
 	name VARCHAR(128) UNIQUE NOT NULL,
+	update_time TIMESTAMPTZ,
 	task_load FLOAT CHECK (NOT task_load < 0) NOT NULL DEFAULT 0.0,
 	ready BOOLEAN NOT NULL DEFAULT 'false'
 ) WITHOUT OIDS;