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/fixtures/customer_fixtures.py

@@ -9,7 +9,7 @@ See tests/conftest.py for details on fixture best practices.
 import pytest
 
 from app.modules.customers.models.customer import Customer, CustomerAddress
-from models.database.order import Order
+from app.modules.orders.models import Order
 
 
 @pytest.fixture
@@ -145,7 +145,7 @@ def test_order(db, test_vendor, test_customer, test_customer_address):
 @pytest.fixture
 def test_order_item(db, test_order, test_product):
     """Create a test order item."""
-    from models.database.order import OrderItem
+    from app.modules.orders.models import OrderItem
 
     order_item = OrderItem(
         order_id=test_order.id,