feat: add short_code field to device login API

- Add optional short_code parameter to DeviceLoginRequest
- Add ShortCode field to Device model
- Save short_code to database during device registration
- Add database migration for user_device.short_code column
- Fix duplicate variable declaration in routes.go

initialize/migrate/database/02129_device_short_code.up.sql

@@ -0,0 +1,2 @@
+-- Add short_code column to user_device table
+ALTER TABLE `user_device` ADD COLUMN `short_code` VARCHAR(255) DEFAULT '' COMMENT 'Short Code' AFTER `identifier`;