[AZ-534] TOTP-based 2FA at credential login

Add RFC 6238 TOTP enrollment, two-step /login flow, recovery codes, and
the amr=["pwd","mfa"] claim that propagates through refresh-token rotation.

- New endpoints: /users/me/mfa/{enroll,confirm,disable} and /login/mfa.
- /login short-circuits to a 5-min ES256 step-1 token (audience-pinned
  azaion-mfa-step2) when the user has MFA enabled; real access+refresh
  pair is minted only after /login/mfa.
- mfa_secret encrypted at rest via ASP.NET Core IDataProtector
  (purpose=Azaion.Mfa.Secret.v1; key folder configurable via
  DataProtection:KeysFolder for production persistence).
- Recovery codes (10 single-use, base32, ~80-bit entropy) hashed with
  SHA-256 and stored as JSONB; constant-time compare on lookup.
- RFC 6238 §5.2 replay defense via mfa_last_used_window per user.
- Sessions carry mfa_authenticated so /token/refresh re-stamps the
  amr claim correctly across the entire 30-day refresh window.
- New audit events: enroll, confirm, disable, login-success/failed,
  recovery-used.
- Schema: env/db/10_users_mfa.sql adds users.mfa_* columns and
  sessions.mfa_authenticated; mfa_recovery_codes mapped as BinaryJson
  in AzaionDbSchemaHolder; disable path uses raw parameterised SQL to
  avoid LinqToDB null-literal type-inference on jsonb columns.

E2E: 6 new tests in MfaLoginTests cover all six AC; full suite
82 passed / 0 failed / 3 intentional skips.

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

Azaion.Services/AuditLog.cs

@@ -11,6 +11,14 @@ public interface IAuditLog
     Task RecordLoginLockout(string email, CancellationToken ct = default);
     Task RecordLoginSuccess(string email, CancellationToken ct = default);
 
+    // AZ-534 — MFA lifecycle + login auth-event audit.
+    Task RecordMfaEnroll       (string email, CancellationToken ct = default);
+    Task RecordMfaConfirm      (string email, CancellationToken ct = default);
+    Task RecordMfaDisable      (string email, CancellationToken ct = default);
+    Task RecordMfaLoginSuccess (string email, CancellationToken ct = default);
+    Task RecordMfaLoginFailed  (string email, CancellationToken ct = default);
+    Task RecordMfaRecoveryUsed (string email, CancellationToken ct = default);
+
     /// <summary>
     /// Number of `login_failed` rows for the given email within the last <paramref name="windowSeconds"/>.
     /// Used by the per-account sliding-window rate limit (AZ-537 AC-2).
@@ -29,6 +37,19 @@ public class AuditLog(IDbFactory dbFactory, IHttpContextAccessor httpContextAcce
     public Task RecordLoginSuccess(string email, CancellationToken ct = default)
         => Insert(AuditEventTypes.LoginSuccess, email, ct);
 
+    public Task RecordMfaEnroll       (string email, CancellationToken ct = default)
+        => Insert(AuditEventTypes.MfaEnroll,        email, ct);
+    public Task RecordMfaConfirm      (string email, CancellationToken ct = default)
+        => Insert(AuditEventTypes.MfaConfirm,       email, ct);
+    public Task RecordMfaDisable      (string email, CancellationToken ct = default)
+        => Insert(AuditEventTypes.MfaDisable,       email, ct);
+    public Task RecordMfaLoginSuccess (string email, CancellationToken ct = default)
+        => Insert(AuditEventTypes.MfaLoginSuccess,  email, ct);
+    public Task RecordMfaLoginFailed  (string email, CancellationToken ct = default)
+        => Insert(AuditEventTypes.MfaLoginFailed,   email, ct);
+    public Task RecordMfaRecoveryUsed (string email, CancellationToken ct = default)
+        => Insert(AuditEventTypes.MfaRecoveryUsed,  email, ct);
+
     public async Task<int> CountRecentFailedLogins(string email, int windowSeconds, CancellationToken ct = default)
     {
         var cutoff = DateTime.UtcNow.AddSeconds(-windowSeconds);