add abandoned cart recovery

When a Stripe checkout session expires without payment, if the customer
entered their email, we record an AbandonedCart and schedule a single
plain-text recovery email (1h delay via Oban).

Privacy design:
- feature is off by default; shop owner opts in via admin settings
- only contacts customers who entered their email at Stripe checkout
- single email, never more (emailed_at timestamp gate)
- suppression list blocks repeat contact; one-click unsubscribe via
  signed token (/unsubscribe/:token)
- records pruned after 30 days (nightly Oban cron)
- no tracking pixels, no redirected links, no HTML

Legal notes:
- custom_text added to Stripe session footer when recovery is on
- UK PECR soft opt-in; EU legitimate interests both satisfied by this design

Files:
- migration: abandoned_carts + email_suppressions tables
- schemas: AbandonedCart, EmailSuppression
- context: Orders.create_abandoned_cart, check_suppression, add_suppression,
  has_recent_paid_order?, get_abandoned_cart_by_session, mark_abandoned_cart_emailed
- workers: AbandonedCartEmailWorker (checkout queue), AbandonedCartPruneWorker (cron)
- notifier: OrderNotifier.deliver_cart_recovery/3
- webhook: extended checkout.session.expired handler
- controller: UnsubscribeController, admin settings toggle
- tests: 28 new tests across context, workers, and controller

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

lib/berrypod_web/controllers/checkout_controller.ex

@@ -68,6 +68,7 @@ defmodule BerrypodWeb.CheckoutController do
         }
       }
       |> maybe_add_shipping_options(hydrated_items)
+      |> maybe_add_cart_recovery_notice()
 
     case Stripe.Checkout.Session.create(params) do
       {:ok, session} ->
@@ -94,6 +95,19 @@ defmodule BerrypodWeb.CheckoutController do
     end
   end
 
+  defp maybe_add_cart_recovery_notice(params) do
+    if Berrypod.Settings.abandoned_cart_recovery_enabled?() do
+      Map.put(params, :custom_text, %{
+        after_submit: %{
+          message:
+            "If your payment doesn't complete, we may send you one follow-up email. You can unsubscribe at any time."
+        }
+      })
+    else
+      params
+    end
+  end
+
   defp maybe_add_shipping_options(params, hydrated_items) do
     gb_result = Shipping.calculate_for_cart(hydrated_items, "GB")
     us_result = Shipping.calculate_for_cart(hydrated_items, "US")