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-Frontend/src/lib/audit-log/labels.ts

@@ -9,6 +9,7 @@ const AUDIT_ACTION_LABELS: Record<string, string> = {
   TOKEN_REFRESHED: "Token erneuert",
   MFA_ENABLED: "MFA aktiviert",
   MFA_DISABLED: "MFA deaktiviert",
+  ACCOUNT_LOCKED: "Konto vorübergehend gesperrt",
 };
 
 export function auditActionLabel(action: string): string {