Implement completion endpoint for allowance (#55)

Closes #19

Reviewed-on: #55

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 1.0,
+    weight real not null default 0.0,
     balance integer not null default 0
 ) strict;