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_order.py

@@ -6,7 +6,7 @@ from datetime import datetime, timezone
 import pytest
 from sqlalchemy.exc import IntegrityError
 
-from models.database.order import Order, OrderItem
+from app.modules.orders.models import Order, OrderItem
 
 
 def create_order_with_snapshots(