fix: remove is_page filter from cart page hydration

Posts used as containers aren't necessarily pages. Market and events
apps don't filter on is_page either.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app.py

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