add persistent email session for order lookup and reviews

Replaces the short-lived (1 hour) session-based order lookup with a
persistent cookie-based email session lasting 30 days. This foundation
enables customers to leave reviews and view orders without re-verifying
their email each time.

- Add EmailSession module for signed cookie management
- Add EmailSession plug to load verified email into session
- Set email session on order lookup verification
- Set email session on checkout completion (via /checkout/complete)
- Update orders and order detail pages to use email session
- Add reviews system plan document

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/berrypod_web/controllers/checkout_controller.ex

@@ -71,7 +71,7 @@ defmodule BerrypodWeb.CheckoutController do
       %{
         mode: "payment",
         line_items: line_items,
-        success_url: R.url(R.checkout_success()) <> "?session_id={CHECKOUT_SESSION_ID}",
+        success_url: R.url("/checkout/complete") <> "?session_id={CHECKOUT_SESSION_ID}",
         cancel_url: R.url(R.cart()),
         metadata: %{"order_id" => order.id},
         shipping_address_collection: %{