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/account-guard.service.ts

@@ -0,0 +1,97 @@
+import { Injectable, Logger } from '@nestjs/common';
+import { PrismaService } from '../../prisma/prisma.service';
+import { AuditLogService } from '../../audit/audit-log.service';
+import { MfaService } from './mfa.service';
+
+/** After this many failed attempts in a row the account is locked ... */
+export const MAX_FAILED_AUTH_ATTEMPTS = 5;
+/** ... for this long. */
+export const AUTH_LOCK_MS = 15 * 60 * 1000;
+
+export interface GuardedUser {
+  id: string;
+  email: string;
+  mfaSecret: string | null;
+  mfaLastTimeStep: number | null;
+  authLockedUntil: Date | null;
+}
+
+/**
+ * Account-level brute-force protection, on top of the per-IP throttle:
+ * an attacker spreading guesses over many addresses still only gets five
+ * tries per account per 15 minutes - for the password, the MFA code and
+ * step-up confirmations alike. A TOTP code is accepted once: its time step
+ * is recorded, and that step or any earlier one is refused afterwards.
+ */
+@Injectable()
+export class AccountGuardService {
+  private readonly logger = new Logger(AccountGuardService.name);
+
+  constructor(
+    private readonly prisma: PrismaService,
+    private readonly mfaService: MfaService,
+    private readonly auditLog: AuditLogService,
+  ) {}
+
+  isLocked(user: Pick<GuardedUser, 'authLockedUntil'>): boolean {
+    return Boolean(user.authLockedUntil && user.authLockedUntil > new Date());
+  }
+
+  async recordFailure(userId: string): Promise<void> {
+    const updated = await this.prisma.user.update({
+      where: { id: userId },
+      data: { failedAuthAttempts: { increment: 1 } },
+      select: { failedAuthAttempts: true },
+    });
+    if (updated.failedAuthAttempts >= MAX_FAILED_AUTH_ATTEMPTS) {
+      await this.prisma.user.update({
+        where: { id: userId },
+        data: {
+          authLockedUntil: new Date(Date.now() + AUTH_LOCK_MS),
+          failedAuthAttempts: 0,
+        },
+      });
+      this.logger.warn(`Account ${userId} locked after repeated failures`);
+      await this.auditLog.record({
+        entityType: 'AuthSession',
+        entityId: userId,
+        action: 'ACCOUNT_LOCKED',
+        userId,
+      });
+    }
+  }
+
+  async recordSuccess(userId: string): Promise<void> {
+    await this.prisma.user.update({
+      where: { id: userId },
+      data: { failedAuthAttempts: 0, authLockedUntil: null },
+    });
+  }
+
+  /**
+   * True only for a valid code whose time step is newer than the last one
+   * used - and only for one of two concurrent requests with the same code.
+   */
+  async consumeTotp(user: GuardedUser, code: string): Promise<boolean> {
+    if (!user.mfaSecret) {
+      return false;
+    }
+    const timeStep = await this.mfaService.verifyCodeStep(
+      user.mfaSecret,
+      user.email,
+      code,
+      user.mfaLastTimeStep ?? undefined,
+    );
+    if (timeStep === null) {
+      return false;
+    }
+    const claimed = await this.prisma.user.updateMany({
+      where: {
+        id: user.id,
+        OR: [{ mfaLastTimeStep: null }, { mfaLastTimeStep: { lt: timeStep } }],
+      },
+      data: { mfaLastTimeStep: timeStep },
+    });
+    return claimed.count === 1;
+  }
+}