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>

app/controllers/api/v1/base_controller.rb

@@ -6,6 +6,9 @@ class Api::V1::BaseController < ApplicationController
   # Skip regular session-based authentication for API
   skip_authentication
 
+  # Skip CSRF protection for API endpoints
+  skip_before_action :verify_authenticity_token
+
   # Skip onboarding requirements for API endpoints
   skip_before_action :require_onboarding_and_upgrade