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>

lib/berrypod_web/endpoint.ex

@@ -8,7 +8,8 @@ defmodule BerrypodWeb.Endpoint do
     store: :cookie,
     key: "_berrypod_key",
     signing_salt: "JNwRcD7y",
-    same_site: "Lax"
+    same_site: "Lax",
+    max_age: 604_800
   ]
 
   socket "/live", Phoenix.LiveView.Socket,