fix: string coercion for email verify code

Some databases will automatically do this string coercion, but others
will not. This commit ensures the value is always a string.

src/backend/src/routers/signup.js

@@ -343,7 +343,7 @@ module.exports = eggspress(['/signup'], {
                 // uuid
                 user_uuid,
                 // email_confirm_code
-                email_confirm_code,
+                '' + email_confirm_code,
                 // email_confirm_token
                 email_confirm_token,
                 // email_confirmed