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/integration/api/v1/admin/test_inventory.py

@@ -447,7 +447,7 @@ class TestAdminInventoryAPI:
     ):
         """Test admin deleting inventory entry."""
         # Create a new inventory entry to delete
-        from models.database.inventory import Inventory
+        from app.modules.inventory.models import Inventory
 
         new_inventory = Inventory(
             product_id=test_product.id,

tests/integration/api/v1/admin/test_letzshop.py

@@ -248,7 +248,7 @@ class TestAdminLetzshopOrdersAPI:
         """Test listing vendor orders with data."""
         from datetime import datetime, timezone
 
-        from models.database.order import Order
+        from app.modules.orders.models import Order
 
         # Create test order using unified Order model with all required fields
         order = Order(

tests/integration/api/v1/admin/test_messages.py

@@ -7,7 +7,7 @@ Tests the /api/v1/admin/messages/* endpoints.
 
 import pytest
 
-from models.database.message import ConversationType, ParticipantType
+from app.modules.messaging.models import ConversationType, ParticipantType
 
 
 @pytest.mark.integration

tests/integration/api/v1/admin/test_order_item_exceptions.py

@@ -9,8 +9,8 @@ All endpoints require admin JWT authentication.
 import pytest
 from datetime import datetime, timezone
 
-from models.database.order import OrderItem
-from models.database.order_item_exception import OrderItemException
+from app.modules.orders.models import OrderItem
+from app.modules.orders.models import OrderItemException
 
 
 @pytest.fixture