fix(auth): lock accounts after failed attempts, one-time TOTP codes, stronger MFA changes

- Per-account lockout: five failed attempts (password, MFA code or a
  step-up confirmation) lock the account for 15 minutes, on top of the
  per-IP throttle - guesses spread over many addresses no longer help.
- A TOTP code is accepted once: the time step is stored and that step or
  any earlier one is refused (also between two concurrent requests).
- Login runs bcrypt on every path and answers unknown email, wrong
  password, deactivated and locked accounts with the same message; failed
  logins for unknown emails no longer write attacker-chosen text into the
  audit log.
- Turning MFA off needs password and a current code; turning it on needs
  the password. Both end all other sessions.
- New audit action ACCOUNT_LOCKED.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

ApiCore-Backend/prisma/migrations/20261001100000_auth_lockout_totp_replay/migration.sql

@@ -0,0 +1,5 @@
+-- TOTP replay protection and per-account lockout after failed attempts.
+ALTER TABLE "User" ADD COLUMN "mfaLastTimeStep" INTEGER;
+ALTER TABLE "User" ADD COLUMN "failedAuthAttempts" INTEGER NOT NULL DEFAULT 0;
+ALTER TABLE "User" ADD COLUMN "authLockedUntil" TIMESTAMP(3);
+ALTER TYPE "AuditAction" ADD VALUE 'ACCOUNT_LOCKED';