[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/AuthService.cs

@@ -16,8 +16,10 @@ public interface IAuthService
     /// AZ-531 / AZ-532 — mint a 15-minute ES256 access token. <paramref name="sessionId"/>
     /// is stamped as the <c>sid</c> claim (logout / family-revocation key in AZ-535)
     /// and <paramref name="jti"/> is the per-token unique id (AZ-535 access denylist).
+    /// AZ-534 — <paramref name="amr"/> values are stamped as repeated <c>amr</c>
+    /// claims so verifiers can require step-up MFA. Defaults to <c>["pwd"]</c>.
     /// </summary>
-    AccessToken CreateToken(User user, Guid sessionId, Guid jti);
+    AccessToken CreateToken(User user, Guid sessionId, Guid jti, IEnumerable<string>? amr = null);
 }
 
 public sealed record AccessToken(string Jwt, DateTime ExpiresAt);
@@ -42,22 +44,31 @@ public class AuthService(
         return await userService.GetByEmail(email);
     }
 
-    public AccessToken CreateToken(User user, Guid sessionId, Guid jti)
+    public AccessToken CreateToken(User user, Guid sessionId, Guid jti, IEnumerable<string>? amr = null)
     {
         var active = signingKeys.Active;
         var signingCredentials = new SigningCredentials(active.SecurityKey, SecurityAlgorithms.EcdsaSha256);
 
         var expires = DateTime.UtcNow.AddMinutes(_jwt.AccessTokenLifetimeMinutes);
+        var claims = new List<Claim>
+        {
+            new(ClaimTypes.NameIdentifier,           user.Id.ToString()),
+            new(ClaimTypes.Name,                     user.Email),
+            new(ClaimTypes.Role,                     user.Role.ToString()),
+            new(JwtRegisteredClaimNames.Sid,         sessionId.ToString()),
+            new(JwtRegisteredClaimNames.Jti,         jti.ToString())
+        };
+
+        // AZ-534 — stamp authentication-methods-reference per RFC 8176. Multi-valued:
+        // password+TOTP login produces ["pwd","mfa"]; recovery-code login adds "recovery".
+        var amrValues = amr?.ToArray() ?? ["pwd"];
+        foreach (var v in amrValues)
+            claims.Add(new Claim("amr", v));
+
         var tokenHandler = new JwtSecurityTokenHandler();
         var tokenDescriptor = new SecurityTokenDescriptor
         {
-            Subject = new ClaimsIdentity([
-                new Claim(ClaimTypes.NameIdentifier, user.Id.ToString()),
-                new Claim(ClaimTypes.Name, user.Email),
-                new Claim(ClaimTypes.Role, user.Role.ToString()),
-                new Claim(JwtRegisteredClaimNames.Sid, sessionId.ToString()),
-                new Claim(JwtRegisteredClaimNames.Jti, jti.ToString())
-            ]),
+            Subject            = new ClaimsIdentity(claims),
             Expires            = expires,
             Issuer             = _jwt.Issuer,
             Audience           = _jwt.Audience,