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/.env.example

@@ -9,6 +9,16 @@ PORT=3001
 # Generate with: openssl rand -base64 32
 JWT_SECRET=your-very-secure-jwt-secret-at-least-32-characters-long-example123456
 
+# Admin 2FA (TOTP) secret encryption key — OPTIONAL.
+# Admin authenticator secrets are encrypted at rest (AES-256-GCM). By default
+# the key is derived from JWT_SECRET, so you do NOT need to set this. Set it
+# only if you want the MFA encryption key decoupled from JWT_SECRET (e.g. so
+# rotating JWT_SECRET doesn't invalidate enrolled authenticators). If you set
+# it, changing/losing it makes existing 2FA secrets undecryptable — recover
+# with: docker compose exec backend node scripts/reset-admin-mfa.js --all --yes
+# Generate with: openssl rand -base64 32
+#MFA_ENCRYPTION_KEY=
+
 # Auth cookie Secure flag
 #   unset  - default: 'auto' in production, false in dev (#427)
 #   true   - always set Secure (HTTPS-only cookies; breaks plain-HTTP access —