feat: extract machine name from heartbeat requests (resolve #47)

migrations/sqlite3/4_machine_column.sql

@@ -0,0 +1,11 @@
+-- +migrate Up
+-- SQL in section 'Up' is executed when this migration is applied
+
+alter table users
+    add `machine` varchar(255);
+
+-- +migrate Down
+-- SQL section 'Down' is executed when this migration is rolled back
+
+alter table users
+    drop column `machine`;