refactor: update admin/vendor API imports to use module locations

Update all admin and vendor API routes to import schemas from
canonical module locations instead of legacy re-export files:
- messages: use app.modules.messaging.models/schemas
- customers: use app.modules.customers.schemas
- orders: use app.modules.orders.schemas
- inventory: use app.modules.inventory.schemas
- invoices: use app.modules.orders.schemas

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/api/v1/admin/orders.py

@@ -21,7 +21,7 @@ from app.api.deps import get_current_admin_api
 from app.core.database import get_db
 from app.services.order_service import order_service
 from models.database.user import User
-from models.schema.order import (
+from app.modules.orders.schemas import (
     AdminOrderItem,
     AdminOrderListResponse,
     AdminOrderStats,