Fix async SQLAlchemy MissingGreenlet error on campaign update

Refresh campaign object after flush to reload expired attributes
and relationships, preventing lazy load failures in async context.

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

backend/app/repositories/campaign_repository.py

@@ -90,6 +90,8 @@ class CampaignRepository:
                 setattr(campaign, key, value)
 
         await self.session.flush()
+        # Refresh to reload expired attributes and relationships after flush
+        await self.session.refresh(campaign, ["agency", "proofs"])
         return campaign
 
     async def delete(self, campaign_id: uuid.UUID) -> bool: