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/schema.prisma

@@ -187,6 +187,7 @@ model User {
   lastActiveAt       DateTime?
   totpSecret         String?           // Base32 TOTP secret
   totpEnabled        Boolean  @default(false)
+  lastTotpAt         DateTime?         // tracks last successful TOTP use to prevent replay
   isActive           Boolean  @default(true)
   deletedAt          DateTime?