List in-flight delivery retries

Scheduled next-attempts (delayed), runnable (waiting), and in-progress (active) retry jobs for the current tenant. Jobs are scoped to bindings the API key's customer owns. For a delayed job, next_attempt_at is when the backoff elapses; for waiting/active jobs it is null (runnable now). Bounded to the most recent jobs per state — best-effort under deep backlogs.