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/vendor_fixtures.py

@@ -11,7 +11,7 @@ import uuid
 import pytest
 
 from models.database.company import Company
-from models.database.inventory import Inventory
+from app.modules.inventory.models import Inventory
 from models.database.product import Product
 from models.database.vendor import Vendor