Fix MissingGreenlet 500 error on GET /api/campaigns

Add selectinload(Campaign.agency) to get_with_proof_counts query so the
agency relationship is eagerly loaded. Without it, accessing campaign.agency.name
in the route triggered a lazy load in an async context, raising
sqlalchemy.exc.MissingGreenlet and returning HTTP 500.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/repositories/campaign_repository.py

@@ -114,6 +114,7 @@ class CampaignRepository:
                 func.count(Proof.id).label("proof_count"),
             )
             .outerjoin(Proof)
+            .options(selectinload(Campaign.agency))
             .group_by(Campaign.id)
         )
         if agency_id: