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>

README.md

@@ -191,6 +191,12 @@ A monitor goes down after the set number of failures in a row (default 2), raise
 escalation policy, and recovers on the first success. Monitors marked public appear on `/status`, together with
 incidents marked public. The status page never shows staff names or private monitors.
 
+The status page shows each public monitor as 90 daily uptime bars, like most hosted status pages. A day is green with
+no failed checks, amber when at least 99% of checks passed, red below that, and grey with no data. Hovering a bar
+(or tapping it on a phone) gives the date, the uptime and the number of failed checks. Days are UTC. Counts are kept
+in a small per-day table, so the bars go back 90 days even though raw check results are kept for
+`SANDBAR_CHECK_DAYS` only.
+
 Monitors cannot probe private or internal addresses unless you tick **Allow targets on private networks** on that
 monitor, so a user who can add a monitor cannot use Sandbar to scan your network.