When a remote compute is unreachable, the controller now uses exponential
backoff for reconnection attempts: 5s, 10s, 20s, 40s, 80s, then caps at
300s (5 minutes). Previously it retried every 5 seconds indefinitely.
User-initiated operations (open project, start node) still trigger an
immediate connection attempt, so recovery is not delayed in practice.
Related: #2704