fix: pass material_override through when creating order lines

create_order and add_order_line endpoints were not passing
material_override from the request body to the OrderLine constructor.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/routers/orders.py

@@ -390,6 +390,7 @@ async def create_order(
             render_position_id=line_data.render_position_id,
             global_render_position_id=line_data.global_render_position_id,
             gewuenschte_bildnummer=line_data.gewuenschte_bildnummer,
+            material_override=line_data.material_override,
             notes=line_data.notes,
             tenant_id=getattr(user, 'tenant_id', None),
         )
@@ -834,6 +835,7 @@ async def add_order_line(
         render_position_id=body.render_position_id,
         global_render_position_id=body.global_render_position_id,
         gewuenschte_bildnummer=body.gewuenschte_bildnummer,
+        material_override=body.material_override,
         notes=body.notes,
         tenant_id=getattr(user, 'tenant_id', None),
     )