feat(status): last poll clock on the status board

[CodeDotJS]

Summary

• Worker stores last successful bake dispatch in KV; public GET returns polledAt
• Status board shows polled beside checked, with 12-hour times and quiet chips
• Docs/rules allow one STATUS_POLL KV for this stamp

Test plan

• Status shows closed / open / checked / polled chips on phone and laptop
• Times use AM/PM
• Offline: checked still works; polled may be empty
• GET https://detro-status-ping.rushforlinux.workers.dev/ returns { polledAt }
• Preview routes load