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/bars_test.go

@@ -0,0 +1,88 @@
+package sandbar
+
+import (
+	"encoding/json"
+	"errors"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+)
+
+// Every check lands in a per-day count that outlives the raw checks, and the status page
+// gets one [checks, failed] pair per day, oldest first and ending today.
+func TestUptimeBars(t *testing.T) {
+	s, clk, _ := newTestServer(t)
+	m := &Monitor{ID: "m", Name: "Site", Kind: "http", Target: "http://x", Enabled: true, Public: true, Retries: 1}
+	s.st.put("monitor", m.ID, m)
+
+	// Two days ago: 4 checks, 1 failed. Yesterday: nothing. Today: 2 checks, both fine.
+	clk.Advance(-48 * time.Hour)
+	for i := 0; i < 4; i++ {
+		var err error
+		if i == 3 {
+			err = errors.New("timeout")
+		}
+		s.recordResult(m, s.monitorState(m.ID), clk.Now(), 10*time.Millisecond, err)
+	}
+	clk.Advance(48 * time.Hour)
+	s.recordResult(m, s.monitorState(m.ID), clk.Now(), 10*time.Millisecond, nil)
+	s.recordResult(m, s.monitorState(m.ID), clk.Now(), 10*time.Millisecond, nil)
+
+	days := s.dailyUptime(m.ID, 5, clk.Now())
+	want := [][2]int{{0, 0}, {0, 0}, {4, 1}, {0, 0}, {2, 0}}
+	for i := range want {
+		if days[i] != want[i] {
+			t.Fatalf("days = %v, want %v", days, want)
+		}
+	}
+
+	// The rollup survives pruning the raw checks.
+	s.st.prune(clk.Now().AddDate(0, 0, 60), 30, 1)
+	if got := s.dailyUptime(m.ID, 5, clk.Now()); got[2] != [2]int{4, 1} {
+		t.Fatalf("rollup lost after pruning: %v", got)
+	}
+
+	ts := httptest.NewServer(s.Handler())
+	defer ts.Close()
+	resp, err := http.Get(ts.URL + "/status.json")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer resp.Body.Close()
+	var out struct {
+		Today    string
+		Monitors []struct {
+			Uptime float64
+			Days   [][2]int
+		}
+	}
+	json.NewDecoder(resp.Body).Decode(&out)
+	if out.Today != "2026-01-05" || len(out.Monitors) != 1 || len(out.Monitors[0].Days) != uptimeDays {
+		t.Fatalf("status.json: %+v", out)
+	}
+	m0 := out.Monitors[0]
+	if last := m0.Days[uptimeDays-1]; last != [2]int{2, 0} {
+		t.Fatalf("today's bar: %v", last)
+	}
+	if want := 100 * 5.0 / 6.0; m0.Uptime < want-0.01 || m0.Uptime > want+0.01 {
+		t.Fatalf("uptime %.2f, want %.2f", m0.Uptime, want)
+	}
+}
+
+// A database from before the daily table gets it built from the checks it has.
+func TestDailyBackfill(t *testing.T) {
+	s, clk, _ := newTestServer(t)
+	now := clk.Now().Unix()
+	s.st.db.Exec(`INSERT INTO checks(monitor_id,time,ok,ms,detail) VALUES('m',?,1,5,''),('m',?,0,5,'x')`, now, now+1)
+	s.st.db.Exec(`DELETE FROM daily`)
+	var have int
+	s.st.db.QueryRow(`SELECT count(*) FROM daily`).Scan(&have)
+	if have != 0 {
+		t.Fatal("setup")
+	}
+	s.st.backfillDaily()
+	if got := s.dailyUptime("m", 1, clk.Now()); got[0] != [2]int{2, 1} {
+		t.Fatalf("backfill: %v", got)
+	}
+}