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/src/auth/mfa/totp.util.ts

@@ -20,6 +20,23 @@ export function buildTotpUri(secret: string, label: string): string {
   return createTotp(secret, label).toURI();
 }
 
+/**
+ * The time step a code belongs to, or null if it is wrong - or not newer
+ * than afterTimeStep (a code that was already used, or an older one).
+ */
+export async function verifyTotpStep(
+  secret: string,
+  label: string,
+  code: string,
+  afterTimeStep?: number,
+): Promise<number | null> {
+  const result = await createTotp(secret, label).verify(code, {
+    epochTolerance: EPOCH_TOLERANCE_SECONDS,
+    afterTimeStep,
+  });
+  return result.valid ? result.timeStep : null;
+}
+
 export async function verifyTotpCode(
   secret: string,
   label: string,