[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/RefreshTokenService.cs

@@ -18,9 +18,10 @@ public interface IRefreshTokenService
     /// <summary>
     /// Mint a fresh refresh token at login; starts a new session family. Returns
     /// the opaque token (NEVER persisted; only its sha256 lands in the DB) and
-    /// the session row that backs it.
+    /// the session row that backs it. <paramref name="mfaAuthenticated"/> is pinned
+    /// to the session so refresh-token rotation inherits the original AMR strength.
     /// </summary>
-    Task<(string OpaqueToken, Session Session)> IssueForNewLogin(Guid userId, CancellationToken ct = default);
+    Task<(string OpaqueToken, Session Session)> IssueForNewLogin(Guid userId, bool mfaAuthenticated = false, CancellationToken ct = default);
 
     /// <summary>
     /// Rotate <paramref name="opaqueToken"/>. On success returns the new token +
@@ -37,20 +38,21 @@ public class RefreshTokenService(IDbFactory dbFactory, IOptions<SessionConfig> s
 
     private readonly SessionConfig _cfg = sessionConfig.Value;
 
-    public async Task<(string OpaqueToken, Session Session)> IssueForNewLogin(Guid userId, CancellationToken ct = default)
+    public async Task<(string OpaqueToken, Session Session)> IssueForNewLogin(Guid userId, bool mfaAuthenticated = false, CancellationToken ct = default)
     {
         var (opaque, hash) = GenerateToken();
         var now = DateTime.UtcNow;
         var session = new Session
         {
-            Id              = Guid.NewGuid(),
-            UserId          = userId,
-            RefreshHash     = hash,
-            FamilyId        = Guid.NewGuid(), // self-rooted family
-            IssuedAt        = now,
-            LastUsedAt      = now,
-            ExpiresAt       = now.AddHours(_cfg.RefreshSlidingHours),
-            FamilyStartedAt = now,
+            Id               = Guid.NewGuid(),
+            UserId           = userId,
+            RefreshHash      = hash,
+            FamilyId         = Guid.NewGuid(), // self-rooted family
+            IssuedAt         = now,
+            LastUsedAt       = now,
+            ExpiresAt        = now.AddHours(_cfg.RefreshSlidingHours),
+            FamilyStartedAt  = now,
+            MfaAuthenticated = mfaAuthenticated,
         };
         // family_id should equal id for the root row so SELECT family_id from
         // any row returns a stable handle even if id is renamed later.
@@ -104,15 +106,16 @@ public class RefreshTokenService(IDbFactory dbFactory, IOptions<SessionConfig> s
             var (newOpaque, newHash) = GenerateToken();
             var newSession = new Session
             {
-                Id              = Guid.NewGuid(),
-                UserId          = current.UserId,
-                RefreshHash     = newHash,
-                FamilyId        = current.FamilyId,
-                IssuedAt        = now,
-                LastUsedAt      = now,
-                ExpiresAt       = now.AddHours(_cfg.RefreshSlidingHours),
-                FamilyStartedAt = current.FamilyStartedAt,
-                ParentSessionId = current.Id,
+                Id               = Guid.NewGuid(),
+                UserId           = current.UserId,
+                RefreshHash      = newHash,
+                FamilyId         = current.FamilyId,
+                IssuedAt         = now,
+                LastUsedAt       = now,
+                ExpiresAt        = now.AddHours(_cfg.RefreshSlidingHours),
+                FamilyStartedAt  = current.FamilyStartedAt,
+                ParentSessionId  = current.Id,
+                MfaAuthenticated = current.MfaAuthenticated,
             };
 
             await db.Sessions