Add secure OAuth2-based mobile authentication

- Replace API keys with OAuth2 tokens for mobile apps
- Add device tracking and management for mobile sessions
- Implement 30-day token expiration with refresh tokens
- Add MFA/2FA support for mobile login
- Create dedicated auth endpoints (signup/login/refresh)
- Skip CSRF protection for API endpoints
- Return plaintext tokens (not hashed) in responses
- Track devices with unique IDs and metadata
- Enable seamless native mobile experience without OAuth redirects

This provides enterprise-grade security for the iOS/Android apps while maintaining a completely native authentication flow.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

config/initializers/doorkeeper.rb

@@ -255,7 +255,7 @@ Doorkeeper.configure do
   # NOTE: you must also run the rails g doorkeeper:application_owner generator
   # to provide the necessary support
   #
-  # enable_application_owner confirmation: false
+  enable_application_owner confirmation: false
 
   # Define access token scopes for your provider
   # For more information go to