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/api.go

@@ -802,10 +802,11 @@ func (s *Server) heartbeat(w http.ResponseWriter, r *http.Request) {
 // statusJSON feeds the public status page: only monitors and incidents marked public.
 func (s *Server) statusJSON(w http.ResponseWriter, r *http.Request) {
 	type mon struct {
-		Name   string  `json:"name"`
-		Status string  `json:"status"`
-		Uptime float64 `json:"uptime"`
-		Since  int64   `json:"since"`
+		Name   string   `json:"name"`
+		Status string   `json:"status"`
+		Uptime float64  `json:"uptime"`
+		Since  int64    `json:"since"`
+		Days   [][2]int `json:"days"` // [checks, failed] per UTC day, oldest first, ending today
 	}
 	type inc struct {
 		Title    string  `json:"title"`
@@ -819,14 +820,24 @@ func (s *Server) statusJSON(w http.ResponseWriter, r *http.Request) {
 		Monitors  []mon  `json:"monitors"`
 		Incidents []inc  `json:"incidents"`
 		Overall   string `json:"overall"`
-	}{[]mon{}, []inc{}, "operational"}
+		Today     string `json:"today"` // the UTC date of the last bar
+	}{[]mon{}, []inc{}, "operational", s.now().UTC().Format("2006-01-02")}
 	mons, _ := listOf[Monitor](s.st, "monitor")
 	for _, m := range mons {
 		if !m.Public || !m.Enabled {
 			continue
 		}
 		st := s.monitorState(m.ID)
-		out.Monitors = append(out.Monitors, mon{m.Name, st.Status, st.Uptime30, st.Since})
+		days := s.dailyUptime(m.ID, uptimeDays, s.now())
+		var checks, fails int
+		for _, d := range days {
+			checks, fails = checks+d[0], fails+d[1]
+		}
+		uptime := -1.0 // no checks yet
+		if checks > 0 {
+			uptime = float64(checks-fails) * 100 / float64(checks)
+		}
+		out.Monitors = append(out.Monitors, mon{m.Name, st.Status, uptime, st.Since, days})
 		if st.Status == "down" {
 			out.Overall = "outage"
 		}