refactor: product independence - remove inheritance pattern

Change Product/ProductTranslation from "override/inheritance" pattern
(NULL = inherit from marketplace) to "independent copy" pattern
(all fields populated at creation).

Key changes:
- Remove OVERRIDABLE_FIELDS, effective_* properties, reset_* methods
- Rename get_override_info() → get_source_comparison_info()
- Update copy_to_vendor_catalog() to copy ALL fields + translations
- Replace effective_* with direct field access in services
- Remove *_overridden fields from schema, keep *_source for comparison
- Add migration to populate NULL fields from marketplace products

The marketplace_product_id FK is kept for "view original source" feature.
Rollback tag: v1.0.0-pre-product-independence

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/services/cart_service.py

@@ -176,8 +176,8 @@ class CartService:
 
         # Get current price in cents (use sale_price if available, otherwise regular price)
         current_price_cents = (
-            product.effective_sale_price_cents
-            or product.effective_price_cents
+            product.sale_price_cents
+            or product.price_cents
             or 0
         )