[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/BusinessException.cs

@@ -68,6 +68,21 @@ public enum ExceptionEnum
     [Description("Aircraft not found or wrong role.")]
     AircraftNotFound = 55,
 
+    [Description("MFA is already enabled for this user.")]
+    MfaAlreadyEnabled = 56,
+
+    [Description("MFA enrollment is not in progress for this user.")]
+    MfaNotEnrolling = 57,
+
+    [Description("MFA is not enabled for this user.")]
+    MfaNotEnabled = 58,
+
+    [Description("Invalid MFA code or recovery code.")]
+    InvalidMfaCode = 59,
+
+    [Description("MFA token is invalid or expired.")]
+    InvalidMfaToken = 61,
+
     [Description("No file provided.")]
     NoFileProvided = 60,
 }