fix(poller): add 64MB cap on DEVICE_EVENTS NATS stream

Without a byte limit, the stream grows unbounded within its 24h
max_age window. At 101 devices polling every 60s, it hits 128MB
in ~10 hours and OOMs the NATS container.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

poller/internal/bus/publisher.go

@@ -141,6 +141,8 @@ func NewPublisher(natsURL string) (*Publisher, error) {
 			"audit.session.end.>",
 		},
 		MaxAge:   24 * time.Hour,
+		MaxBytes: 64 * 1024 * 1024, // 64MB cap — discard oldest when full
+		Discard:  jetstream.DiscardOld,
 	})
 	if err != nil {
 		nc.Close()