fix: authenticate unwatched hosts during the status probe (#1337)

With metrics enabled, the status probe left SSH authentication to the
metrics poll - which only runs while someone is viewing the host. An
unwatched host therefore never left "reachable", while a host with
metrics disabled (whose probe always authenticates) showed online. The
probe now authenticates whenever no metrics poll will.

src/backend/hosts/metrics/host-status.ts

@@ -15,3 +15,18 @@ export function statusAfterAuthentication(
   if (authenticated) return "online";
   return current === "offline" ? "offline" : "reachable";
 }
+
+/**
+ * Whether the cheap status probe must also authenticate over SSH.
+ *
+ * "online" means authenticated, and normally the metrics poll proves that.
+ * That poll only runs while someone is viewing the host, so an unwatched host
+ * with metrics enabled would otherwise never leave "reachable" - while a host
+ * with metrics disabled, whose probe always authenticates, shows online.
+ */
+export function needsStatusPollAuthentication(
+  metricsEnabled: boolean,
+  hasViewers: boolean,
+): boolean {
+  return !metricsEnabled || !hasViewers;
+}