Show 90-day uptime bars on the status page

Every check is also counted in a per-day table that outlives the raw checks, so the public page
can draw one bar per UTC day with a tooltip, as most hosted status pages do. Existing databases
are backfilled once from the checks they still have.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docs/design.md

@@ -28,6 +28,10 @@ The helpers at the top of the script in `index.html` are the same as Hammerhead'
 | `rowList(addLabel, make, initial)` | A list the person adds to and removes from, such as people in a rotation or steps in a policy |
 | `timeline(events)` | Dated entries for an alert or an incident |
 
+## Status page bars
+
+* **Uptime bars** on the public page are one bar per UTC day, 90 of them. Green is no failures, amber is at least 99% passing, red is below that, grey is no data. Color is never the only signal: the row carries a text status badge and the uptime percentage, each bar's tooltip says the date and the numbers in words, and the whole strip has an `aria-label` summary.
+
 ## Icons
 
 Lucide style, outline only, stroke 1.75, round caps, `currentColor`. Sizes are 16, 18 and 22 pixels. They are inline in the `ICONS` table. An icon goes with a text label, and icon-only buttons carry an `aria-label`.