feat: add Stripe checkout, order persistence, and webhook handling

Stripe-hosted Checkout integration with full order lifecycle:
- stripity_stripe ~> 3.2 with sandbox/prod config via env vars
- Order and OrderItem schemas with price snapshots at purchase time
- CheckoutController creates pending order then redirects to Stripe
- StripeWebhookController verifies signatures and confirms payment
- Success page with real-time PubSub updates from webhook
- Shop flash messages for checkout error feedback
- Cart cleared after successful payment

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

lib/simpleshop_theme/orders/order.ex

@@ -0,0 +1,48 @@
+defmodule SimpleshopTheme.Orders.Order do
+  use Ecto.Schema
+  import Ecto.Changeset
+
+  @primary_key {:id, :binary_id, autogenerate: true}
+  @foreign_key_type :binary_id
+
+  @payment_statuses ~w(pending paid failed refunded)
+
+  schema "orders" do
+    field :order_number, :string
+    field :stripe_session_id, :string
+    field :stripe_payment_intent_id, :string
+    field :payment_status, :string, default: "pending"
+    field :customer_email, :string
+    field :shipping_address, :map, default: %{}
+    field :subtotal, :integer
+    field :total, :integer
+    field :currency, :string, default: "gbp"
+    field :metadata, :map, default: %{}
+
+    has_many :items, SimpleshopTheme.Orders.OrderItem
+
+    timestamps(type: :utc_datetime)
+  end
+
+  def changeset(order, attrs) do
+    order
+    |> cast(attrs, [
+      :order_number,
+      :stripe_session_id,
+      :stripe_payment_intent_id,
+      :payment_status,
+      :customer_email,
+      :shipping_address,
+      :subtotal,
+      :total,
+      :currency,
+      :metadata
+    ])
+    |> validate_required([:order_number, :subtotal, :total, :currency])
+    |> validate_inclusion(:payment_status, @payment_statuses)
+    |> validate_number(:subtotal, greater_than_or_equal_to: 0)
+    |> validate_number(:total, greater_than_or_equal_to: 0)
+    |> unique_constraint(:order_number)
+    |> unique_constraint(:stripe_session_id)
+  end
+end

lib/simpleshop_theme/orders/order_item.ex

@@ -0,0 +1,27 @@
+defmodule SimpleshopTheme.Orders.OrderItem do
+  use Ecto.Schema
+  import Ecto.Changeset
+
+  @primary_key {:id, :binary_id, autogenerate: true}
+  @foreign_key_type :binary_id
+
+  schema "order_items" do
+    field :variant_id, :string
+    field :product_name, :string
+    field :variant_title, :string
+    field :quantity, :integer
+    field :unit_price, :integer
+
+    belongs_to :order, SimpleshopTheme.Orders.Order
+
+    timestamps(type: :utc_datetime)
+  end
+
+  def changeset(item, attrs) do
+    item
+    |> cast(attrs, [:variant_id, :product_name, :variant_title, :quantity, :unit_price, :order_id])
+    |> validate_required([:variant_id, :product_name, :quantity, :unit_price])
+    |> validate_number(:quantity, greater_than: 0)
+    |> validate_number(:unit_price, greater_than_or_equal_to: 0)
+  end
+end