[GFI] Add Authy Two-Factor Authentication

**Description:**
Introduce Authy (or similar) as an additional two-factor authentication option.

**Tasks:**
- Integrate Authy API for device-based 2FA.
- Replace or augment existing SMS/email 2FA with proper provider implementation.
- Update settings page to allow users to enable/disable Authy 2FA.
- Ensure secure 2FA challenge on login.

**Acceptance Criteria:**
- Users can register Authy for their account.
- Login flow includes 2FA challenge when enabled.
- Existing 2FA via SMS/email still works as fallback.
- All 2FA actions are logged securely.

**References:**
- Related models: `User`, `Setting`.
- Review current 2FA logic for compatibility.

Start by reviewing the current 2FA logic and the related User and Setting models. Trace the settings and login flows, then compare them with the acceptance criteria: Authy registration and challenge, SMS/email fallback, and secure action logging.

Written by the indexing model from the issue text.