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>

test/berrypod_web/live/shop/orders_test.exs

@@ -14,7 +14,7 @@ defmodule BerrypodWeb.Shop.OrdersTest do
   defp with_lookup_email(conn, email) do
     conn
     |> Phoenix.ConnTest.init_test_session(%{})
-    |> Plug.Conn.put_session("order_lookup_email", email)
+    |> Plug.Conn.put_session("email_session", email)
   end
 
   describe "orders list — no session email" do