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>

config/initializers/rack_attack.rb

@@ -83,12 +83,17 @@ class Rack::Attack
   end
 
   # ### Prevent Brute-Force Login Attacks ###
+  # Exclude MFA verification attempts from regular login throttling
   throttle('login/ip', limit: 5, period: 5.minutes) do |req|
-    req.ip if req.path_without_extentions == '/auth/sign_in' && req.post?
+    if req.path_without_extentions == '/auth/sign_in' && req.post? && req.params['mfa_token'].blank?
+      # Skip if this is an MFA verification request
+      req.ip
+    end
   end
 
   throttle('login/email', limit: 10, period: 15.minutes) do |req|
-    if req.path_without_extentions == '/auth/sign_in' && req.post?
+    # Skip if this is an MFA verification request
+    if req.path_without_extentions == '/auth/sign_in' && req.post? && req.params['mfa_token'].blank?
       # ref: https://github.com/rack/rack-attack/issues/399
       # NOTE: This line used to throw ArgumentError /rails/action_mailbox/sendgrid/inbound_emails : invalid byte sequence in UTF-8
       # Hence placed in the if block
@@ -114,6 +119,28 @@ class Rack::Attack
     req.ip if req.path_without_extentions == '/api/v1/profile/resend_confirmation' && req.post?
   end
 
+  ## MFA throttling - prevent brute force attacks
+  throttle('mfa_verification/ip', limit: 5, period: 1.minute) do |req|
+    if req.path_without_extentions == '/api/v1/profile/mfa'
+      req.ip if req.delete? # Throttle disable attempts
+    elsif req.path_without_extentions.match?(%r{/api/v1/profile/mfa/(verify|backup_codes)})
+      req.ip if req.post? # Throttle verify and backup_codes attempts
+    end
+  end
+
+  # Separate rate limiting for MFA verification attempts
+  throttle('mfa_login/ip', limit: 10, period: 1.minute) do |req|
+    req.ip if req.path_without_extentions == '/auth/sign_in' && req.post? && req.params['mfa_token'].present?
+  end
+
+  throttle('mfa_login/token', limit: 10, period: 1.minute) do |req|
+    if req.path_without_extentions == '/auth/sign_in' && req.post?
+      # Track by MFA token to prevent brute force on a specific token
+      mfa_token = req.params['mfa_token'].presence
+      (mfa_token.presence)
+    end
+  end
+
   ## Prevent Brute-Force Signup Attacks ###
   throttle('accounts/ip', limit: 5, period: 30.minutes) do |req|
     req.ip if req.path_without_extentions == '/api/v1/accounts' && req.post?