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/models/mobile_device.rb

@@ -0,0 +1,55 @@
+class MobileDevice < ApplicationRecord
+  belongs_to :user
+  belongs_to :oauth_application, class_name: "Doorkeeper::Application", optional: true
+
+  validates :device_id, presence: true, uniqueness: { scope: :user_id }
+  validates :device_name, presence: true
+  validates :device_type, presence: true, inclusion: { in: %w[ios android] }
+
+  before_validation :set_last_seen_at, on: :create
+
+  scope :active, -> { where("last_seen_at > ?", 90.days.ago) }
+
+  def active?
+    last_seen_at > 90.days.ago
+  end
+
+  def update_last_seen!
+    update_column(:last_seen_at, Time.current)
+  end
+
+  def create_oauth_application!
+    return oauth_application if oauth_application.present?
+
+    app = Doorkeeper::Application.create!(
+      name: "Mobile App - #{device_id}",
+      redirect_uri: "maybe://oauth/callback", # Custom scheme for mobile
+      scopes: "read_write", # Use the configured scope
+      confidential: false # Public client for mobile
+    )
+    
+    # Store the association
+    update!(oauth_application: app)
+    app
+  end
+
+  def active_tokens
+    return Doorkeeper::AccessToken.none unless oauth_application
+    
+    Doorkeeper::AccessToken
+      .where(application: oauth_application)
+      .where(resource_owner_id: user_id)
+      .where(revoked_at: nil)
+      .where("expires_in IS NULL OR created_at + expires_in * interval '1 second' > ?", Time.current)
+  end
+
+  def revoke_all_tokens!
+    active_tokens.update_all(revoked_at: Time.current)
+  end
+
+  private
+
+    def set_last_seen_at
+      self.last_seen_at ||= Time.current
+    end
+end