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>

Gemfile.lock

@@ -212,6 +212,11 @@ GEM
       railties (>= 4.1.0)
       responders
       warden (~> 1.2.3)
+    devise-two-factor (6.1.0)
+      activesupport (>= 7.0, < 8.1)
+      devise (~> 4.0)
+      railties (>= 7.0, < 8.1)
+      rotp (~> 6.0)
     devise_token_auth (1.2.5)
       bcrypt (~> 3.0)
       devise (> 3.5.2, < 5)
@@ -723,6 +728,7 @@ GEM
     reverse_markdown (2.1.1)
       nokogiri
     rexml (3.4.1)
+    rotp (6.3.0)
     rspec-core (3.13.0)
       rspec-support (~> 3.13.0)
     rspec-expectations (3.13.2)
@@ -1005,6 +1011,7 @@ DEPENDENCIES
   debug (~> 1.8)
   devise (>= 4.9.4)
   devise-secure_password!
+  devise-two-factor (>= 5.0.0)
   devise_token_auth (>= 1.2.3)
   dotenv-rails (>= 3.0.0)
   down