refactor: update test imports to use module locations

Update all test files to import from canonical module locations:
- Integration tests: orders, inventory, messages, invoices
- Unit tests: services and models
- Fixtures: customer, vendor, message fixtures

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

tests/unit/services/test_stats_service.py

@@ -9,7 +9,7 @@ from sqlalchemy.exc import SQLAlchemyError
 
 from app.exceptions import AdminOperationException, VendorNotFoundException
 from app.services.stats_service import StatsService
-from models.database.inventory import Inventory
+from app.modules.inventory.models import Inventory
 from models.database.marketplace_product import MarketplaceProduct
 from models.database.marketplace_product_translation import (
     MarketplaceProductTranslation,