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>

.env.example

@@ -6,6 +6,13 @@
 # Use `rake secret` to generate this variable
 SECRET_KEY_BASE=replace_with_lengthy_secure_hex
 
+# Active Record Encryption keys (required for MFA/2FA functionality)
+# Generate these keys by running: rails db:encryption:init
+# IMPORTANT: Use different keys for each environment (development, staging, production)
+# ACTIVE_RECORD_ENCRYPTION_PRIMARY_KEY=
+# ACTIVE_RECORD_ENCRYPTION_DETERMINISTIC_KEY=
+# ACTIVE_RECORD_ENCRYPTION_KEY_DERIVATION_SALT=
+
 # Replace with the URL you are planning to use for your app
 FRONTEND_URL=http://0.0.0.0:3000
 # To use a dedicated URL for help center pages