Fix cart route missing slug param (popped by app-level preprocessor)

App-level url_value_preprocessor pops slug from values, so the
product blueprint's cart() never receives it. Use g.product_slug
(set by resolve_product before_request) instead.

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

bp/product/routes.py

@@ -159,7 +159,8 @@ def register():
 
     @bp.post("/cart/")
     @clear_cache(tag="browse", tag_scope="user")
-    async def cart(slug: str):
+    async def cart(**_kw):
+        slug = g.product_slug
         # make sure product exists (we *allow* deleted_at != None later if you want)
         product_id = await g.s.scalar(
             select(Product.id).where(