/admin/data's Health section charts Pi temperature, with under-voltage minutes marked on the same axis #546

The page half of #532.

Done when:

• [ ] The Health section of /admin/data draws Pi temperature over the selected range, with under-voltage minutes as marks on the same axis, and exact values on hover or tap. GET /data carries the series.
• [ ] Tests: the series comes through /data, and an empty range draws no chart.
• [ ] bun --cwd Software/so101 test passes.

Brief: any builder.