Tolerate transient peer connection failures in Oban leadership

Hive now tolerates transient database connection failures during Oban leader election instead of crashing the peer process. A local database peer implementation logs exhausted election transactions and schedules the next election, while leadership is dropped locally to avoid a stale replica continuing to act as leader. Production database connections also enable client socket keepalives with a short Linux probe cadence, reducing dead idle sockets. There is no migration or operator action required.