[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>

Azaion.Common/Entities/AuditEvent.cs

@@ -16,6 +16,12 @@ public static class AuditEventTypes
     public const string LoginLockout      = "login_lockout";
     public const string LoginSuccess      = "login_success";
 
+    // AZ-556 — per-category internal forensics for unified `InvalidCredentials` wire
+    // response. SecOps can distinguish these in the audit_events table even though the
+    // /login response cannot be distinguished by an attacker.
+    public const string LoginFailedUnknownEmail = "login_failed_unknown_email";
+    public const string LoginFailedDisabled     = "login_failed_disabled";
+
     // AZ-534 — MFA lifecycle + login events.
     public const string MfaEnroll         = "mfa_enroll";
     public const string MfaConfirm        = "mfa_confirm";