Prune expired terminal github_webhook_delivery rows (ignored 7d, processed 30d).

Its OWN route, deliberately not folded into github-webhooks/reconcile: that call's budget is the review critical path and has already starved twice. Bounded per call (batched deletes inside a wall-clock budget) and a strict no-op once the backlog is pruned, so it is safe on every tick. Never touches queued or failed.