feat: MFA (#12290)

## Linear:
- https://github.com/chatwoot/chatwoot/issues/486

## Description
This PR implements Multi-Factor Authentication (MFA) support for user
accounts, enhancing security by requiring a second form of verification
during login. The feature adds TOTP (Time-based One-Time Password)
authentication with QR code generation and backup codes for account
recovery.

## Type of change

- [ ] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

- Added comprehensive RSpec tests for MFA controller functionality
- Tested MFA setup flow with QR code generation
- Verified OTP validation and backup code generation
- Tested login flow with MFA enabled/disabled

## Checklist:

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

---------

Co-authored-by: Pranav <pranav@chatwoot.com>
Co-authored-by: Sojan Jose <sojan@pepalo.com>
Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

app/services/mfa/token_service.rb

@@ -0,0 +1,28 @@
+class Mfa::TokenService < BaseTokenService
+  pattr_initialize [:user, :token]
+
+  MFA_TOKEN_EXPIRY = 5.minutes
+
+  def generate_token
+    @payload = build_payload
+    super
+  end
+
+  def verify_token
+    decoded = decode_token
+    return nil if decoded.blank?
+
+    User.find(decoded[:user_id])
+  rescue ActiveRecord::RecordNotFound
+    nil
+  end
+
+  private
+
+  def build_payload
+    {
+      user_id: user.id,
+      exp: MFA_TOKEN_EXPIRY.from_now.to_i
+    }
+  end
+end