[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.Common/Entities/Session.cs

@@ -43,6 +43,13 @@ public class Session
     /// interactive sessions.
     /// </summary>
     public Guid?     AircraftId        { get; set; }
+
+    /// <summary>
+    /// AZ-534 — true iff the session was created via an MFA-validated /login/mfa
+    /// call. Refresh-token rotation reads this to keep the AMR claim stable across
+    /// the session lifetime.
+    /// </summary>
+    public bool      MfaAuthenticated  { get; set; }
 }
 
 public static class SessionRevokedReasons