Revert "fix: remove is_page filter from cart page hydration"

This reverts commit d0da418f12.

app.py

@@ -120,7 +120,7 @@ def create_app() -> "Quart":
             return
         post = (
             await g.s.execute(
-                select(Post).where(Post.slug == slug)
+                select(Post).where(Post.slug == slug, Post.is_page == True)  # noqa: E712
             )
         ).scalar_one_or_none()
         if not post: