feat(11-04): add the websockets health, VAPID key and test-push commands

- centrifugo.Client.Info probes the info API method; an error body fails
- websockets:health ports CentrifugoHealthCheck: exits 1 without an API
  key or when the probe fails, prints the Setting/Value table otherwise
- websockets:generate-vapid-keys prints a new P-256 pair, shows configured
  keys only truncated, and --update persists them to overrides.yaml
- websockets:test-push reads subscriptions from an app-published
  SubscriptionSource, refuses to send while push is disabled and sends
  one encrypted push per subscription
- no command prints a configured private key or the Centrifugo API key
- flare and lighthouse READMEs document the CLI commands

modules/lighthouse/centrifugo/client.go

@@ -158,6 +158,37 @@ func (c *Client) Unsubscribe(ctx context.Context, userID uint, channel string) e
 	return err
 }
 
+// Info POSTs {} to {api_url}/info and returns its result: Centrifugo's node
+// list and statistics. It is the connectivity probe of websockets:health.
+// Unlike the publishing calls, an answer with an error body is an error.
+func (c *Client) Info(ctx context.Context) (map[string]any, error) {
+	out := map[string]any{}
+	if !c.Enabled() {
+		return out, ErrNotConfigured
+	}
+	raw, err := c.post(ctx, "/info", struct{}{})
+	if err != nil {
+		return out, err
+	}
+	var resp struct {
+		Error *struct {
+			Code    int    `json:"code"`
+			Message string `json:"message"`
+		} `json:"error"`
+		Result map[string]any `json:"result"`
+	}
+	if err := json.Unmarshal(raw, &resp); err != nil {
+		return out, fmt.Errorf("centrifugo: /info: response is not JSON")
+	}
+	if resp.Error != nil {
+		return out, fmt.Errorf("centrifugo: /info: error %d: %s", resp.Error.Code, resp.Error.Message)
+	}
+	if resp.Result != nil {
+		out = resp.Result
+	}
+	return out, nil
+}
+
 func (c *Client) data(event string, payload json.RawMessage) eventData {
 	if len(bytes.TrimSpace(payload)) == 0 {
 		payload = json.RawMessage("[]")