feat(auth): admin TOTP MFA — enrollment, login challenge, recovery, CLI reset

Backend for #738. Real TOTP 2FA for admin accounts, all roles incl.
super_admin (closes #735).

- mfaService: otplib TOTP; AES-256-GCM encryption of the secret at rest
  (key derived from MFA_ENCRYPTION_KEY or JWT_SECRET); bcrypt-hashed,
  single-use recovery codes; otpauth URI + QR.
- Migration 151: adds two_factor_recovery_codes + two_factor_enrolled_at
  (secret/enabled columns already existed from legacy 016).
- Enrollment endpoints (behind adminAuth, per-user): GET /mfa/status,
  POST /mfa/{setup,enable,disable,recovery-codes}. Disable/regenerate
  require a current code so a hijacked session can't strip 2FA.
- Login challenge: /admin/login returns {mfaRequired, mfaToken} (no
  session) when 2FA is on; /admin/login/mfa exchanges a TOTP or recovery
  code for the session. Lockout counter is NOT reset until the second
  factor passes, so MFA brute-force is rate-limited too.
- CLI break-glass: scripts/reset-admin-mfa.js --email <e> | --all --yes,
  audit-logged, matches reset-admin-password.js convention.
- Docs + optional MFA_ENCRYPTION_KEY env.

Verified end-to-end on a live backend: enroll (super_admin), challenge,
TOTP + single-use recovery login, disable, and CLI reset.

backend/migrations/core/151_add_admin_mfa_recovery.js

@@ -0,0 +1,58 @@
+/**
+ * Migration 151: admin MFA (TOTP) enrollment support — issue #738.
+ *
+ * The `admin_users.two_factor_enabled` / `two_factor_secret` columns already
+ * exist from the legacy migration 016 but were never wired to any code. This
+ * migration adds the two columns the real TOTP flow needs on top of them:
+ *
+ *   - two_factor_recovery_codes: JSON array of one-time backup codes, stored
+ *     HASHED (never plaintext), so a locked-out admin can log in without the
+ *     authenticator. Consumed on use.
+ *   - two_factor_enrolled_at: when the admin completed enrollment (audit /
+ *     display only).
+ *
+ * The TOTP secret itself continues to live in the existing `two_factor_secret`
+ * column, but is now stored ENCRYPTED at rest (AES-256-GCM) by mfaService —
+ * the column type is unchanged (the encrypted blob is short).
+ *
+ * Additive and idempotent: only adds columns, guarded by hasColumn, so it is
+ * safe to re-run and touches no existing data.
+ */
+exports.up = async function (knex) {
+  const hasRecovery = await knex.schema.hasColumn('admin_users', 'two_factor_recovery_codes');
+  const hasEnrolledAt = await knex.schema.hasColumn('admin_users', 'two_factor_enrolled_at');
+  const hasEnabled = await knex.schema.hasColumn('admin_users', 'two_factor_enabled');
+  const hasSecret = await knex.schema.hasColumn('admin_users', 'two_factor_secret');
+
+  await knex.schema.alterTable('admin_users', (t) => {
+    // Backfill the legacy columns too, in case an install somehow lacks them
+    // (016 is a legacy migration; guard defensively).
+    if (!hasEnabled) {
+      t.boolean('two_factor_enabled').defaultTo(false);
+    }
+    if (!hasSecret) {
+      t.string('two_factor_secret').nullable();
+    }
+    if (!hasRecovery) {
+      t.text('two_factor_recovery_codes').nullable();
+    }
+    if (!hasEnrolledAt) {
+      t.timestamp('two_factor_enrolled_at').nullable();
+    }
+  });
+};
+
+exports.down = async function (knex) {
+  const hasRecovery = await knex.schema.hasColumn('admin_users', 'two_factor_recovery_codes');
+  const hasEnrolledAt = await knex.schema.hasColumn('admin_users', 'two_factor_enrolled_at');
+
+  await knex.schema.alterTable('admin_users', (t) => {
+    // Only drop what THIS migration added; leave the legacy 016 columns.
+    if (hasRecovery) {
+      t.dropColumn('two_factor_recovery_codes');
+    }
+    if (hasEnrolledAt) {
+      t.dropColumn('two_factor_enrolled_at');
+    }
+  });
+};