Add "drop table if exists" for exchange table

The initialization script is only called if no database exists, so this
shouldn't matter since there would be nothing to drop,
but consistency is nice.

server/init.sql

@@ -5,6 +5,7 @@ drop table if exists transactions;
 drop table if exists accounts_transactions;
 drop table if exists notifications;
 drop table if exists users_notifications;
+drop table if exists exchange;
 
 create table users (
 	id integer primary key autoincrement,