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

db/migrate/20250916230001_add_index_to_payouts_on_event_id_and_status.rb

@@ -0,0 +1,5 @@
+class AddIndexToPayoutsOnEventIdAndStatus < ActiveRecord::Migration[7.1]
+  def change
+    add_index :payouts, [ :event_id, :status ]
+  end
+end

db/migrate/20250916230002_add_index_to_earnings_on_status.rb

@@ -0,0 +1,5 @@
+class AddIndexToEarningsOnStatus < ActiveRecord::Migration[7.1]
+  def change
+    add_index :earnings, :status
+  end
+end

db/migrate/20250916230003_add_index_to_tickets_on_status_and_order_id.rb

@@ -0,0 +1,5 @@
+class AddIndexToTicketsOnStatusAndOrderId < ActiveRecord::Migration[7.1]
+  def change
+    add_index :tickets, [ :status, :order_id ]
+  end
+end