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/shipping/shipping_rate.ex

@@ -0,0 +1,53 @@
+defmodule SimpleshopTheme.Shipping.ShippingRate do
+  use Ecto.Schema
+  import Ecto.Changeset
+
+  @primary_key {:id, :binary_id, autogenerate: true}
+  @foreign_key_type :binary_id
+
+  schema "shipping_rates" do
+    field :blueprint_id, :integer
+    field :print_provider_id, :integer
+    field :country_code, :string
+    field :first_item_cost, :integer
+    field :additional_item_cost, :integer
+    field :currency, :string, default: "USD"
+    field :handling_time_days, :integer
+
+    belongs_to :provider_connection, SimpleshopTheme.Products.ProviderConnection
+
+    timestamps(type: :utc_datetime)
+  end
+
+  def changeset(rate, attrs) do
+    rate
+    |> cast(attrs, [
+      :provider_connection_id,
+      :blueprint_id,
+      :print_provider_id,
+      :country_code,
+      :first_item_cost,
+      :additional_item_cost,
+      :currency,
+      :handling_time_days
+    ])
+    |> validate_required([
+      :provider_connection_id,
+      :blueprint_id,
+      :print_provider_id,
+      :country_code,
+      :first_item_cost,
+      :additional_item_cost,
+      :currency
+    ])
+    |> validate_number(:first_item_cost, greater_than_or_equal_to: 0)
+    |> validate_number(:additional_item_cost, greater_than_or_equal_to: 0)
+    |> foreign_key_constraint(:provider_connection_id)
+    |> unique_constraint([
+      :provider_connection_id,
+      :blueprint_id,
+      :print_provider_id,
+      :country_code
+    ])
+  end
+end