feat(payouts): implement promoter earnings viewing, request flow, and admin Stripe processing with webhooks

Add model methods for accurate net calculations (€0.50 + 1.5% fees), eligibility, refund handling
Update promoter/payouts controller for index (pending events), create (eligibility checks)
Integrate admin processing via Stripe::Transfer, webhook for status sync
Enhance views: index pending cards, events/show preview/form
Add comprehensive tests (models, controllers, service, integration); run migrations

config/routes.rb

@@ -1,6 +1,10 @@
 Rails.application.routes.draw do
   namespace :admin do
-    resources :payouts, only: [ :index, :create ]
+    resources :payouts, only: [ :index, :show ] do
+      member do
+        post :process
+      end
+    end
   end
   # Define your application routes per the DSL in https://guides.rubyonrails.org/routing.html
 
@@ -78,7 +82,7 @@ Rails.application.routes.draw do
 
   # === Promoter Routes ===
   namespace :promoter do
-    resources :payouts, only: [:index, :show, :create]
+    resources :payouts, only: [ :index, :show, :create ]
     resources :events do
       member do
         patch :publish
@@ -115,4 +119,6 @@ Rails.application.routes.draw do
       # resources :ticket_types, only: [ :index, :show, :create, :update, :destroy ]
     end
   end
+
+  post "/webhooks/stripe", to: "webhooks/stripe#create"
 end