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_order_service.py

@@ -28,7 +28,7 @@ from app.services.order_service import (
     order_service,
 )
 from app.modules.customers.models.customer import Customer
-from models.database.order import Order, OrderItem
+from app.modules.orders.models import Order, OrderItem
 
 
 # Default address fields required by Order model
@@ -328,7 +328,7 @@ class TestOrderServiceUpdates:
 
     def test_update_order_status(self, db, test_vendor, test_customer):
         """Test update_order_status changes status"""
-        from models.schema.order import OrderUpdate
+        from app.modules.orders.schemas import OrderUpdate
 
         order = Order(
             vendor_id=test_vendor.id,