Cross-tenant list of in-flight webhook retries (admin)

Surfaces deliveries currently between attempts — status='pending' with a concrete `next_retry_at`. Sorted soonest-due first so the panel highlights the most-urgent items. Each row is enriched with the backoff schedule + remaining-attempt math the UI would otherwise have to re-derive. Hard-capped at 500 rows per call. Sortable: nextRetryAt, attempts. Default sort: nextRetryAt (ascending — soonest-due first).