feat(auth): implement authentication system foundation

Add user authentication infrastructure with login/logout functionality,
session management, and basic user model.

Changes:
- Create users table with email, password_digest, confirmation tokens
- Add Users repository with authentication methods
- Add User entity with authentication helpers
- Implement login form and authentication actions
- Add logout action
- Add BCrypt gem for password hashing
- Update base Action class with authentication helpers
- Add login/logout routes

Next steps: Registration flow, password reset, email confirmation

lib/euchre_camp/entities/user.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+module EuchreCamp
+  module Entities
+    class User < Dry::Struct
+      attribute :id, Types::Integer
+      attribute :player_id, Types::Integer
+      attribute :email, Types::String
+      attribute :password_digest, Types::String
+      attribute :confirmed, Types::Bool
+      attribute? :confirmation_token, Types::String.optional
+      attribute? :confirmation_sent_at, Types::Time.optional
+      attribute? :reset_password_token, Types::String.optional
+      attribute? :reset_password_sent_at, Types::Time.optional
+      attribute :failed_login_attempts, Types::Integer
+      attribute? :locked_until, Types::Time.optional
+      attribute? :last_login_at, Types::Time.optional
+      attribute? :last_login_ip, Types::String.optional
+      attribute :created_at, Types::Time
+      attribute :updated_at, Types::Time
+
+      # Check if account is locked
+      def locked?
+        locked_until && locked_until > Time.now
+      end
+
+      # Check if account is confirmed
+      def active?
+        confirmed && !locked?
+      end
+
+      # Check if user is admin (for navigation purposes)
+      # This would be enhanced with actual role checking later
+      def admin?
+        false  # Placeholder - implement role checking later
+      end
+    end
+  end
+end