Label the actual interval of shared bucket charts

Analytics groups calls in four-hour windows, but the shared DayBars SVG labels every series “per day.” Add an optional interval label with the existing day default so the accessible chart name reflects its actual buckets. The existing packed HTTP consumer verifies the four-hour name in both themes at 1512/390.

Run ID: run_d0299f7ed67c58fd7c3270abc72f522d