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_invoice_service.py

@@ -16,13 +16,13 @@ from app.services.invoice_service import (
     InvoiceService,
     LU_VAT_RATES,
 )
-from models.database.invoice import (
+from app.modules.orders.models import (
     Invoice,
     InvoiceStatus,
     VATRegime,
     VendorInvoiceSettings,
 )
-from models.schema.invoice import (
+from app.modules.orders.schemas import (
     VendorInvoiceSettingsCreate,
     VendorInvoiceSettingsUpdate,
 )