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>

tests/fixtures/store_fixtures.py

@@ -192,7 +192,6 @@ def test_inventory(db, test_product):
         store_id=test_product.store_id,
         warehouse="strassen",
         bin_location=f"SA-10-{unique_id[:2]}",
-        location=f"WAREHOUSE_A_{unique_id}",
         quantity=100,
         reserved_quantity=10,
         gtin=test_product.marketplace_product.gtin,
@@ -213,7 +212,6 @@ def multiple_inventory_entries(db, multiple_products, test_store):
             gtin=product.gtin,
             warehouse="strassen",
             bin_location=f"SA-{i:02d}-01",
-            location=f"LOC_{i}",
             quantity=10 + (i * 5),
             reserved_quantity=i,
             store_id=test_store.id,