fix(security): prevent TOTP replay attacks and fix user enumeration in verifyTotp

Adds lastTotpAt timestamp to User model. After a successful TOTP validation,
the timestamp is recorded. Any reuse of the same code within the 30-second
window is rejected as a replay attack.

verifyTotp now returns a single generic UNAUTHORIZED error regardless of
whether the user ID is invalid or TOTP is not enabled, preventing enumeration
of user IDs and MFA status.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/db/prisma/migrations/20260409_user_last_totp_at.sql

@@ -0,0 +1 @@
+ALTER TABLE "users" ADD COLUMN IF NOT EXISTS "lastTotpAt" TIMESTAMP(3);