prevent checkout with unavailable product variants

- Include is_available flag in hydrated cart items
- Show unavailable message on cart items and product page
- Block add-to-cart for unavailable variants
- Redirect back to cart with error if checkout has unavailable items

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

lib/berrypod_web/controllers/checkout_controller.ex

@@ -16,13 +16,20 @@ defmodule BerrypodWeb.CheckoutController do
       cart_items = Cart.get_from_session(get_session(conn))
       hydrated = Cart.hydrate(cart_items)
 
-      if hydrated == [] do
-        conn
-        |> put_flash(:error, "Your basket is empty")
-        |> redirect(to: ~p"/cart")
-      else
-        track_checkout_start(conn)
-        create_checkout(conn, hydrated)
+      cond do
+        hydrated == [] ->
+          conn
+          |> put_flash(:error, "Your basket is empty")
+          |> redirect(to: ~p"/cart")
+
+        Enum.any?(hydrated, &(&1.is_available == false)) ->
+          conn
+          |> put_flash(:error, "Some items in your basket are no longer available")
+          |> redirect(to: ~p"/cart")
+
+        true ->
+          track_checkout_start(conn)
+          create_checkout(conn, hydrated)
       end
     end
   end