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>

models/schema/vendor_product.py

@@ -25,8 +25,8 @@ class VendorProductListItem(BaseModel):
     vendor_sku: str | None = None
     title: str | None = None
     brand: str | None = None
-    effective_price: float | None = None
-    effective_currency: str | None = None
+    price: float | None = None
+    currency: str | None = None
     is_active: bool | None = None
     is_featured: bool | None = None
     is_digital: bool | None = None
@@ -73,7 +73,11 @@ class CatalogVendorsResponse(BaseModel):
 
 
 class VendorProductDetail(BaseModel):
-    """Detailed vendor product information."""
+    """Detailed vendor product information.
+
+    Products are independent entities - all fields are populated at creation.
+    Source values are kept for "view original source" comparison only.
+    """
 
     id: int
     vendor_id: int
@@ -81,27 +85,22 @@ class VendorProductDetail(BaseModel):
     vendor_code: str | None = None
     marketplace_product_id: int
     vendor_sku: str | None = None
-    # Override info from get_override_info()
+    # Product fields with source comparison
     price: float | None = None
-    price_overridden: bool | None = None
-    price_source: float | None = None
+    price_cents: int | None = None
+    price_source: float | None = None  # For "view original source" feature
     sale_price: float | None = None
-    sale_price_overridden: bool | None = None
+    sale_price_cents: int | None = None
     sale_price_source: float | None = None
     currency: str | None = None
-    currency_overridden: bool | None = None
     currency_source: str | None = None
     brand: str | None = None
-    brand_overridden: bool | None = None
     brand_source: str | None = None
     condition: str | None = None
-    condition_overridden: bool | None = None
     condition_source: str | None = None
     availability: str | None = None
-    availability_overridden: bool | None = None
     availability_source: str | None = None
     primary_image_url: str | None = None
-    primary_image_url_overridden: bool | None = None
     primary_image_url_source: str | None = None
     is_digital: bool | None = None
     product_type: str | None = None