Update default allowance, but differently (#84)

Reviewed-on: #84

backend/migrations/1_initial.sql

@@ -2,7 +2,7 @@ create table users
 (
     id   integer primary key,
     name text not null,
-    weight real not null default 0.0,
+    weight real not null default 10.0,
     balance integer not null default 0
 ) strict;
 

backend/migrations/3_change_weight_default.sql

@@ -1,4 +1 @@
-alter table users rename column weight to weight_old;
-alter table users add column weight real not null default 10.0;
-update users set weight = weight_old;
-alter table users drop column weight_old;
+update users set weight = 10.0 where weight = 0.0;