refactor: remove all backward compatibility code across 70 files

Clean up 28 backward compatibility instances identified in the codebase.
The app is not live, so all shims are replaced with the target architecture:

- Remove legacy Inventory.location column (use bin_location exclusively)
- Remove dashboard _extract_metric_value helper (use flat metrics dict)
- Remove legacy stat field duplicates (total_stores, total_imports, etc.)
- Remove 13 re-export shims and class aliases across modules
- Remove module-enabling JSON fallback (use PlatformModule junction table)
- Remove menu_to_legacy_format() conversion (return dataclasses directly)
- Remove title/description from MarketplaceProductBase schema
- Clean billing convenience method docstrings
- Clean test fixtures and backward-compat comments
- Add PlatformModule seeding to init_production.py

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

app/modules/marketplace/schemas/marketplace_product.py

@@ -1,9 +1,10 @@
 # app/modules/marketplace/schemas/marketplace_product.py
 """Pydantic schemas for MarketplaceProduct API validation.
 
-Note: title and description are stored in MarketplaceProductTranslation table,
-but we keep them in the API schemas for convenience. The service layer
-handles creating/updating translations separately.
+Note: title and description are stored in MarketplaceProductTranslation table.
+The service layer handles creating/updating translations separately via
+dedicated title/description parameters. MarketplaceProductCreate includes
+a required title field for API convenience.
 """
 
 from datetime import datetime
@@ -32,10 +33,6 @@ class MarketplaceProductBase(BaseModel):
 
     marketplace_product_id: str | None = None
 
-    # Localized fields (passed to translations)
-    title: str | None = None
-    description: str | None = None
-
     # Links and media
     link: str | None = None
     image_link: str | None = None