improve cart recovery: product links in email, persistent session cookie

- add product_id to order_items (migration + schema + create_order)
- cart recovery email now includes a direct product link per item
- extend session cookie max_age to 7 days so carts survive browser restarts

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

priv/repo/migrations/20260224000002_add_product_id_to_order_items.exs

@@ -0,0 +1,9 @@
+defmodule Berrypod.Repo.Migrations.AddProductIdToOrderItems do
+  use Ecto.Migration
+
+  def change do
+    alter table(:order_items) do
+      add :product_id, :string
+    end
+  end
+end