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

app/controllers/webhooks/stripe_controller.rb

@@ -0,0 +1,34 @@
+class Webhooks::StripeController < ApplicationController
+  skip_before_action :verify_authenticity_token
+
+  def create
+    payload = request.body.read
+    sig_header = request.env["HTTP_STRIPE_SIGNATURE"]
+
+    begin
+      event = Stripe::Webhook.construct_event(
+        payload, sig_header, ENV["STRIPE_WEBHOOK_SECRET"]
+      )
+    rescue Stripe::SignatureVerificationError => e
+      # Invalid signature
+      return head 400
+    end
+
+    case event["type"]
+    when "transfer.payout.succeeded"
+      payout_id = event.data.object.metadata["payout_id"]
+      payout = Payout.find(payout_id)
+      if payout && payout.processing?
+        payout.update!(status: :completed, stripe_payout_id: event.data.object.id)
+      end
+    when "transfer.payout.failed", "transfer.canceled"
+      payout_id = event.data.object.metadata["payout_id"]
+      payout = Payout.find(payout_id)
+      if payout
+        payout.update!(status: :failed)
+      end
+    end
+
+    head 200
+  end
+end