feat(admin): move allow_update_discount option to product level

shop/supabase/migrations/20260710003000_move_allow_update_discount_to_products.sql

@@ -0,0 +1,7 @@
+-- Migration: Move allow_update_discount from categories to products
+-- Purpose: Control update discount eligibility on product level rather than category level.
+
+ALTER TABLE public.categories DROP COLUMN IF EXISTS allow_update_discount;
+ALTER TABLE public.products ADD COLUMN IF NOT EXISTS allow_update_discount BOOLEAN DEFAULT true NOT NULL;
+
+NOTIFY pgrst, 'reload schema';