fleet-manager: log a positive line when the authorise pillar enables

Every plane logged its state EXCEPT the broker success path, which was silent —
so "authorise pillar working" was indistinguishable from "not logged". Add an
ENABLED line matching the InfluxDB/agent/admin convention.

serving/fleet-manager/main.go

@@ -151,6 +151,7 @@ func main() {
 			log.Fatalf("broker client: %v", err)
 		}
 		s.broker = broker
+		log.Printf("authorise pillar ENABLED -> enrol-broker %s (mTLS)", cfg.brokerAdminURL)
 	}
 
 	// Metrics gateway: the manager is the only reader/writer of InfluxDB. nil (a