add shipping costs with live exchange rates and country detection

Shipping rates fetched from Printify during product sync, converted to
GBP at sync time using frankfurter.app ECB exchange rates with 5%
buffer. Cached in shipping_rates table per blueprint/provider/country.

Cart page shows shipping estimate with country selector (detected from
Accept-Language header, persisted in cookie). Stripe Checkout includes
shipping_options for UK domestic and international delivery. Order
shipping_cost extracted from Stripe on payment.

ScheduledSyncWorker runs every 6 hours via Oban cron to keep rates
and exchange rates fresh. REST_OF_THE_WORLD fallback covers unlisted
countries. 780 tests, 0 failures.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

priv/repo/migrations/20260214004900_create_shipping_rates.exs

@@ -0,0 +1,30 @@
+defmodule SimpleshopTheme.Repo.Migrations.CreateShippingRates do
+  use Ecto.Migration
+
+  def change do
+    create table(:shipping_rates, primary_key: false) do
+      add :id, :binary_id, primary_key: true
+
+      add :provider_connection_id,
+          references(:provider_connections, type: :binary_id, on_delete: :delete_all),
+          null: false
+
+      add :blueprint_id, :integer, null: false
+      add :print_provider_id, :integer, null: false
+      add :country_code, :string, null: false
+      add :first_item_cost, :integer, null: false
+      add :additional_item_cost, :integer, null: false
+      add :currency, :string, null: false, default: "USD"
+      add :handling_time_days, :integer
+
+      timestamps(type: :utc_datetime)
+    end
+
+    create unique_index(:shipping_rates, [
+             :provider_connection_id,
+             :blueprint_id,
+             :print_provider_id,
+             :country_code
+           ])
+  end
+end