refactor: extract cart storage to dedicated API controller with dynamic frontend URLs

- Added dedicated CartsController for session-based cart storage
- Refactored routes to use POST /api/v1/carts/store
- Updated ticket selection JS to use dynamic data attributes for URLs
- Fixed CSRF protection in API and checkout payment increment
- Made checkout button URLs dynamic via data attributes
- Updated tests for new cart storage endpoint
- Removed obsolete store_cart from EventsController

config/routes.rb

@@ -96,11 +96,8 @@ Rails.application.routes.draw do
   namespace :api do
     namespace :v1 do
       # RESTful routes for event management
-      resources :events, only: [ :index, :show, :create, :update, :destroy ] do
-        member do
-          post :store_cart
-        end
-      end
+      resources :events, only: [ :index, :show, :create, :update, :destroy ]
+      post "carts/store", to: "carts#store", as: "store_cart"
 
       # RESTful routes for order management
       resources :orders, only: [] do