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/vendor/test_invoices.py

@@ -10,8 +10,8 @@ from decimal import Decimal
 import pytest
 
 from app.modules.customers.models.customer import Customer
-from models.database.invoice import Invoice, InvoiceStatus, VendorInvoiceSettings
-from models.database.order import Order
+from app.modules.orders.models import Invoice, InvoiceStatus, VendorInvoiceSettings
+from app.modules.orders.models import Order
 
 
 @pytest.mark.integration