The relay's minute sample records pi:temp and pi:undervolt from the arm's GET /health/pi into facts (no chart) #545

The data half of #532. src/facts.ts (5f2aecf) and GET /health/pi (736c70a) are on main.

Done when:

• [ ] The minute sample in src/facts.ts also records pi:temp and pi:undervolt from the arm's GET /health/pi. When the arm is unreachable, slow or answers nulls, nothing is recorded, and the sample never blocks: use a short timeout.
• [ ] Tests: with a fake arm, a sample stores both values. With an unreachable arm, it stores neither and the other samples still land.
• [ ] bun --cwd Software/so101 test passes. No page change.

Brief: any builder, small.