[AZ-556] [AZ-557] Unify login errors + share MFA lockout pipeline

AZ-556 collapses every /login rejection (unknown email, wrong password,
disabled account, lockout, per-account rate limit) to a single opaque
InvalidCredentials (70) → 401 response. Timing equalised by a new
Security.VerifyDummy using the same Argon2id parameters. Audit log keeps
the rejection category internally (login_failed_unknown_email,
login_failed_disabled).

AZ-557 wires /login/mfa into the existing per-account lockout +
rate-limit pipeline. MFA failures now feed UserService's shared failure
accounting (RegisterMfaFailedLogin → RegisterFailedLoginCore) and
CountRecentFailedLogins aggregates both login_failed and
mfa_login_failed rows. Successful TOTP / recovery resets the counter.

Deprecated five legacy ExceptionEnum members (NoEmailFound,
WrongPassword, UserDisabled, AccountLocked, LoginRateLimited) — kept
defined for cross-workspace verifier compatibility during the
deprecation window.

E2E coverage updated: AuthTests (byte-identical body assertion +
disabled-account audit row), LoginRateLimitTests, PasswordHashingTests,
SecurityTests, plus four new MfaLoginTests (AC1, AC2, AC5, AC7).

Code review verdict: PASS_WITH_WARNINGS (batch_06_cycle2_review.md).

Co-authored-by: Cursor <cursoragent@cursor.com>

_docs/_autodev_state.md

@@ -6,11 +6,10 @@ step: 10
 name: Implement
 status: in_progress
 sub_step:
-  phase: 14
-  name: batch-loop
-  detail: "batch 5 done (AZ-552..AZ-555, 6 pts, commit f369153); batch 6 next (AZ-556 unify_login_error_codes + AZ-557 mfa_brute_force_lockout, 5 pts); session boundary — fresh conversation requested by operator"
+  phase: 11
+  name: commit
+  detail: "batch 6 of 6"
   leftovers_to_replay:
-    - _docs/_process_leftovers/2026-05-14_jira_batch5_transitions.md
     - _docs/_process_leftovers/2026-05-14_suite_infra_jwt_secret_drift.md
 retry_count: 0
 cycle: 2