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>

lib/simpleshop_theme/orders/order.ex

@@ -18,6 +18,7 @@ defmodule SimpleshopTheme.Orders.Order do
     field :customer_email, :string
     field :shipping_address, :map, default: %{}
     field :subtotal, :integer
+    field :shipping_cost, :integer
     field :total, :integer
     field :currency, :string, default: "gbp"
     field :metadata, :map, default: %{}
@@ -48,6 +49,7 @@ defmodule SimpleshopTheme.Orders.Order do
       :customer_email,
       :shipping_address,
       :subtotal,
+      :shipping_cost,
       :total,
       :currency,
       :metadata