fix: add missing migration for totp_last_used_counter column

Column existed in Prisma schema but had no migration, causing 500 on
TOTP login in production where the column was absent.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

prisma/migrations/20260428_add_totp_last_used_counter/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE `users` ADD COLUMN `totp_last_used_counter` INTEGER NULL;