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/vendor_product_service.py

@@ -171,9 +171,9 @@ class VendorProductService:
             raise ProductNotFoundException(product_id)
 
         mp = product.marketplace_product
-        override_info = product.get_override_info()
+        source_comparison_info = product.get_source_comparison_info()
 
-        # Get marketplace product translations
+        # Get marketplace product translations (for "view original source")
         mp_translations = {}
         if mp:
             for t in mp.translations:
@@ -183,7 +183,7 @@ class VendorProductService:
                     "short_description": t.short_description,
                 }
 
-        # Get vendor translations (overrides)
+        # Get vendor translations
         vendor_translations = {}
         for t in product.translations:
             vendor_translations[t.language] = {
@@ -198,8 +198,8 @@ class VendorProductService:
             "vendor_code": product.vendor.vendor_code if product.vendor else None,
             "marketplace_product_id": product.marketplace_product_id,
             "vendor_sku": product.vendor_sku,
-            # Override info
-            **override_info,
+            # Product fields with source comparison info
+            **source_comparison_info,
             # Vendor-specific fields
             "is_featured": product.is_featured,
             "is_active": product.is_active,
@@ -270,13 +270,13 @@ class VendorProductService:
             "marketplace_product_id": product.marketplace_product_id,
             "vendor_sku": product.vendor_sku,
             "title": title,
-            "brand": product.effective_brand,
-            "effective_price": product.effective_price,
-            "effective_currency": product.effective_currency,
+            "brand": product.brand,
+            "price": product.price,
+            "currency": product.currency,
             "is_active": product.is_active,
             "is_featured": product.is_featured,
             "is_digital": product.is_digital,
-            "image_url": product.effective_primary_image_url,
+            "image_url": product.primary_image_url,
             "source_marketplace": mp.marketplace if mp else None,
             "source_vendor": mp.vendor_name if mp else None,
             "created_at": product.created_at.isoformat()