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_inventory_service.py

@@ -13,8 +13,8 @@ from app.exceptions import (
     ProductNotFoundException,
 )
 from app.services.inventory_service import InventoryService
-from models.database.inventory import Inventory
-from models.schema.inventory import (
+from app.modules.inventory.models import Inventory
+from app.modules.inventory.schemas import (
     InventoryAdjust,
     InventoryCreate,
     InventoryReserve,