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/models/database/test_inventory.py

@@ -4,7 +4,7 @@
 import pytest
 from sqlalchemy.exc import IntegrityError
 
-from models.database.inventory import Inventory
+from app.modules.inventory.models import Inventory
 
 
 @pytest.mark.unit