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>

internal/sandbar/monitor.go

@@ -233,6 +233,37 @@ func (s *Server) monitorState(id string) MonitorState {
 	return st
 }
 
+// uptimeDays is how many days of bars the status page shows.
+const uptimeDays = 90
+
+// dailyUptime returns [checks, failed] for each of the last n UTC days, oldest first, so the
+// last entry is today. A day with no checks is [0, 0].
+func (s *Server) dailyUptime(monitorID string, n int, now time.Time) [][2]int {
+	out := make([][2]int, n)
+	today := now.UTC().Truncate(24 * time.Hour)
+	rows, err := s.st.db.Query(`SELECT day,checks,fails FROM daily WHERE monitor_id=? AND day>=?`,
+		monitorID, today.AddDate(0, 0, -(n-1)).Format("2006-01-02"))
+	if err != nil {
+		return out
+	}
+	defer rows.Close()
+	for rows.Next() {
+		var day string
+		var c, f int
+		if rows.Scan(&day, &c, &f) != nil {
+			continue
+		}
+		d, err := time.Parse("2006-01-02", day)
+		if err != nil {
+			continue
+		}
+		if i := n - 1 - int(today.Sub(d).Hours()/24); i >= 0 && i < n {
+			out[i] = [2]int{c, f}
+		}
+	}
+	return out
+}
+
 func (s *Server) saveState(st MonitorState) {
 	s.st.db.Exec(`INSERT INTO mstate(monitor_id,status,since,last_check,last_ms,fails,last_ping,detail) VALUES(?,?,?,?,?,?,?,?)
 		ON CONFLICT(monitor_id) DO UPDATE SET status=excluded.status, since=excluded.since, last_check=excluded.last_check,
@@ -309,6 +340,8 @@ func (s *Server) recordResult(m *Monitor, st MonitorState, now time.Time, dur ti
 		okInt = 1
 	}
 	s.st.db.Exec(`INSERT INTO checks(monitor_id,time,ok,ms,detail) VALUES(?,?,?,?,?)`, m.ID, now.Unix(), okInt, dur.Milliseconds(), errText(err))
+	s.st.db.Exec(`INSERT INTO daily(monitor_id,day,checks,fails) VALUES(?,?,1,?)
+		ON CONFLICT(monitor_id,day) DO UPDATE SET checks=checks+1, fails=fails+excluded.fails`, m.ID, now.UTC().Format("2006-01-02"), 1-okInt)
 	st.LastCheck, st.LastMS, st.Detail = now.Unix(), int(dur.Milliseconds()), errText(err)
 	key := "monitor:" + m.ID
 	if ok {
@@ -369,6 +402,7 @@ func (s *Server) Heartbeat(token string) bool {
 func (s *Server) deleteMonitorData(id string) {
 	s.st.db.Exec(`DELETE FROM mstate WHERE monitor_id=?`, id)
 	s.st.db.Exec(`DELETE FROM checks WHERE monitor_id=?`, id)
+	s.st.db.Exec(`DELETE FROM daily WHERE monitor_id=?`, id)
 	s.ResolveAlertKey("monitor:"+id, "monitor deleted")
 }