fix: prevent WebSocket reconnect loops on slow agent collections (#2294)

The hub waited only 5s (the request manager default) for stats over
WebSocket and closed the connection on any error. On hosts where
`zpool list` stalls (seen on Proxmox, up to its 10s timeout), collection
exceeded that limit, so the hub sent a close (code 1000) and the agent
reconnected. The refresh ran every other cycle, which caused a
disconnect roughly every 2 minutes.

Hub:
- Wait up to 30s for WebSocket stats responses.
- Keep the connection open when a stats request times out; only close
  it (and fall back to SSH) for other errors.

Agent:
- After the first collection, refresh `zpool list` pool stats and
  `zfs list` dataset usage in the background and serve cached values
  meanwhile, so a hung utility cannot delay the stats response.

agent/storage_pool_test.go

@@ -518,3 +518,31 @@ func TestBtrfsPoolIdentities(t *testing.T) {
 	assert.Equal(t, first, zm.GetDetail(true).Pools[1].Name)
 	assert.Equal(t, "renamed", zm.GetDetail(true).Pools[1].DisplayName)
 }
+
+func TestStaleUtilityCachesRefreshInBackground(t *testing.T) {
+	release := make(chan struct{})
+	b := &poolBackend{name: "zfs"}
+	b.poolStatsFn = func() ([]zfs.PoolStat, error) {
+		<-release
+		return []zfs.PoolStat{{Name: "new"}}, nil
+	}
+	b.datasetsFn = func() ([]zfs.Dataset, error) {
+		<-release
+		return []zfs.Dataset{{Name: "new", Mountpoint: "/new"}}, nil
+	}
+	b.poolData = []zfs.PoolStat{{Name: "old"}}
+	b.lastPoolStats = time.Now().Add(-2 * poolStatsRefreshInterval)
+	b.datasetUsage = map[string]zfsDatasetUsage{"/old": {}}
+	b.lastUsageRefresh = time.Now().Add(-2 * datasetUsageRefreshInterval)
+
+	// A hung utility must not block collection; cached data is served meanwhile.
+	for range 2 {
+		assert.Equal(t, "old", b.poolStats()[0].Name)
+		assert.Contains(t, b.refreshDatasetUsage(), "/old")
+	}
+
+	close(release)
+	require.Eventually(t, func() bool {
+		return b.poolStats()[0].Name == "new" && b.refreshDatasetUsage()["/new"] == zfsDatasetUsage{}
+	}, time.Second, time.Millisecond)
+}