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/integration/api/v1/admin/test_inventory.py

@@ -447,7 +447,7 @@ class TestAdminInventoryAPI:
     ):
         """Test admin deleting inventory entry."""
         # Create a new inventory entry to delete
-        from models.database.inventory import Inventory
+        from app.modules.inventory.models import Inventory
 
         new_inventory = Inventory(
             product_id=test_product.id,