feat: implement payout system database schema and models

db/migrate/20250916212717_create_earnings.rb

@@ -0,0 +1,16 @@
+class CreateEarnings < ActiveRecord::Migration[8.0]
+  def change
+    create_table :earnings do |t|
+      t.integer :amount_cents
+      t.integer :fee_cents
+      t.integer :status
+      t.string :stripe_payout_id
+
+      t.references :event, null: false, foreign_key: false, index: true
+      t.references :user, null: false, foreign_key: false, index: true
+      t.references :order, null: false, foreign_key: false, index: true
+
+      t.timestamps
+    end
+  end
+end