Fixes the review comments

backend/migrations/2022-04-17-175115_user-system/down.sql

@@ -1,4 +1,4 @@
 -- This file should undo anything in `up.sql`
 
-drop table user;
-drop table pwd;
+drop table pwd;
+drop table user;

backend/migrations/2022-04-17-175115_user-system/up.sql

@@ -6,6 +6,7 @@ CREATE TABLE user (
 );
 
 CREATE TABLE pwd (
-    id INTEGER NOT NULL PRIMARY KEY,
-    password TEXT NOT NULL
+    user_id INTEGER NOT NULL PRIMARY KEY,
+    password TEXT NOT NULL,
+    FOREIGN KEY (user_id) REFERENCES user(id) ON DELETE CASCADE
 );