Compute all widgets over one snapshot (scope: dashboards:read)

Long renders keep the connection warm: the 200 body may begin with whitespace keepalives (newlines, one every ~20s) before the single JSON document — insignificant leading whitespace per RFC 8259, skipped by every standard JSON parser, so res.json()/jq work unchanged. A failure after the response has started arrives as the standard error envelope ({ error: { code, message, request_id } }) in that body; check for an error key, not only the HTTP status.