Admin: leases, pin, release, drain, usage, metrics

cmd/crossbar/main.go calls admin.Handler with the new 6-arg signature, passing nil for the not-yet-wired leases/limiter/store/drainer (task 07 wires them) so go vet and go test ./... pass on cmd/crossbar. This is a compile fix, not the task-07 wiring; noted in the implementer-log.

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

cmd/crossbar/main.go

@@ -49,7 +49,7 @@ func run() error {
 	go table.Run(ctx)
 
 	mux := http.NewServeMux()
-	mux.Handle("/_crossbar/", admin.Handler(cfg, table))
+	mux.Handle("/_crossbar/", admin.Handler(cfg, table, nil, nil, nil, nil))
 	mux.Handle("/", proxy.New(cfg, table, nil, nil, nil, log))
 
 	srv := &http.Server{