[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.Services/Security.cs

@@ -23,6 +23,24 @@ public static class Security
 
     public sealed record VerifyResult(bool Valid, bool NeedsRehash);
 
+    // AZ-556 — timing equalizer for unknown-email and disabled-account branches of
+    // `UserService.ValidateUser`. Pre-computed once with the same Argon2id parameters
+    // as a real hash so a `VerifyDummy(plaintext)` call costs ~the same wall-clock as
+    // a real `VerifyPassword(plaintext, user.PasswordHash)`. The result is always
+    // discarded — this is a side-channel mitigation, not a control-flow path.
+    private static readonly string DummyHashForTiming = HashPassword(
+        "az-556-timing-equalizer-dummy-do-not-store-in-db");
+
+    /// <summary>
+    /// AZ-556 — run the same Argon2id work a real verify would do, then discard the
+    /// result. Used to keep the unknown-email and disabled-account login branches
+    /// timing-indistinguishable from a wrong-password branch.
+    /// </summary>
+    public static void VerifyDummy(string plaintext)
+    {
+        _ = VerifyPassword(plaintext, DummyHashForTiming);
+    }
+
     public static string HashPassword(string plaintext)
     {
         if (plaintext == null) throw new ArgumentNullException(nameof(plaintext));