[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>

_docs/02_tasks/_dependencies_table.md

@@ -1,7 +1,7 @@
 # Dependencies Table
 
-**Date**: 2026-05-14 (post batch 3 cycle 2; previous 2026-05-14)
-**Total Tasks**: 19 (7 done test tasks + 4 done product tasks + 5 done cross-workspace + 3 done CMMC + 4 done auth-modernization + 1 active product task)
+**Date**: 2026-05-14 (post batch 4 cycle 2; previous 2026-05-14)
+**Total Tasks**: 19 (7 done test tasks + 4 done product tasks + 5 done cross-workspace + 3 done CMMC + 5 done auth-modernization)
 **Total Complexity Points**: 71
 
 | Task   | Name                          | Complexity | Dependencies            | Epic   | Status |
@@ -20,7 +20,7 @@
 | AZ-531 | refresh_token_flow            | 5          | None                    | AZ-529 | done   |
 | AZ-532 | asymmetric_signing_jwks       | 5          | None                    | AZ-529 | done   |
 | AZ-533 | mission_token_uav             | 5          | AZ-531                  | AZ-529 | done   |
-| AZ-534 | totp_2fa_login                | 5          | None (coord. AZ-531/537) | AZ-529 | todo   |
+| AZ-534 | totp_2fa_login                | 5          | None (coord. AZ-531/537) | AZ-529 | done   |
 | AZ-535 | logout_revocation             | 3          | AZ-531                  | AZ-529 | done   |
 | AZ-536 | argon2id_password_hashing     | 3          | None                    | AZ-530 | done   |
 | AZ-537 | login_rate_limit_lockout      | 3          | None (coord. AZ-536)    | AZ-530 | done   |