fix: запретить изменение ad_post_url после создания закупа

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

src/usecase/placement/update_placement.py

@@ -26,8 +26,6 @@ async def update_placement(
             placement.cost = input.cost
         if input.comment is not None:
             placement.comment = input.comment
-        if input.ad_post_url is not None:
-            placement.ad_post_url = input.ad_post_url
         if input.status is not None:
             placement.status = input.status