fix: запретить изменение ad_post_url после создания закупа

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

src/dto/placement.py

@@ -60,7 +60,6 @@ class UpdatePlacementInput(pydantic.BaseModel):
     placement_date: datetime.datetime | None = None
     cost: float | None = None
     comment: str | None = None
-    ad_post_url: str | None = None
     status: domain.PlacementStatus | None = None