latest: report each Poll Lane's last pass to the owner's page (#102)

A Lane's pace, its refusal backoff and whether its Site needs the browser were
only ever visible in the log. The admin page (issue #102) has to state them, so
the Poller keeps one snapshot per Lane.

- LaneState/Status (status.go) is the page's view of a Lane: due, gap, clamped,
  refusing, browser.
- runOnce records a snapshot on every return path, including the pass that
  refuses, so a cooling Lane does not read as one that never ran.
- Refusing is derived at snapshot read time from the backoff, not stored: a
  Lane that cooled down between passes must report false without a new pass.
- LaneStatus reports only Sites that have completed a pass, so a fresh restart
  renders "no data yet" instead of confident zeroes.

backend/internal/latest/poller_test.go

@@ -1580,3 +1580,70 @@ func TestRunOnceClampWarningNamesTheSite(t *testing.T) {
 		t.Fatalf("clamp warning = %q, want it to name asura and 3601", got)
 	}
 }
+
+// The owner's admin page (issue #102) reads Lane state out of the poller.
+// Before any pass the snapshot is empty — a restart must render "no data
+// yet", not zeroes — and each pass records what it saw: the frozen clock,
+// the due count and the pace, with refusal backoff derived at snapshot time.
+func TestLaneStatus(t *testing.T) {
+	s, _ := newTestStore(t)
+	now := time.UnixMilli(5_000_000)
+	p := newTestPoller(t, s, &fakeFetcher{body: asuraSeriesFixture, status: 200}, now)
+
+	if st := p.LaneStatus(); len(st.Lanes) != 0 {
+		t.Fatalf("lanes before any pass = %d, want 0 (nothing has run)", len(st.Lanes))
+	} else if st.BrowserConfigured || st.BrowserReachable {
+		t.Fatalf("browser before any pass = configured=%v reachable=%v, want false without a browser fetcher", st.BrowserConfigured, st.BrowserReachable)
+	}
+
+	seedForCheck(t, s, "asura:chronicles", "https://asurascans.com/series/chronicles", 0)
+	// Two refusals put kagane's Lane into backoff; asura sits on its own Lane.
+	browser := &fakeFetcher{status: 403}
+	p.BrowserFetch = browser
+	for i := range 2 {
+		key := fmt.Sprintf("kagane:s%d", i)
+		seedForCheck(t, s, key, "https://kagane.to/series/"+key[7:], 0)
+	}
+	p.runOnce(context.Background())
+
+	st := p.LaneStatus()
+	var asura, kagane LaneState
+	for _, lane := range st.Lanes {
+		switch lane.Site {
+		case "asura":
+			asura = lane
+		case "kagane":
+			kagane = lane
+		}
+	}
+	if st.Lanes[0].Site != "asura" {
+		t.Fatalf("first lane = %q, want asura (snapshot sorted by Site)", st.Lanes[0].Site)
+	}
+	if asura.Site == "" {
+		t.Fatalf("asura missing from snapshot: %+v", st.Lanes)
+	}
+	if !asura.LastRun.Equal(now) {
+		t.Fatalf("asura LastRun = %s, want the frozen clock %s", asura.LastRun, now)
+	}
+	if asura.Due != 1 {
+		t.Fatalf("asura Due = %d, want 1", asura.Due)
+	}
+	if asura.Gap == 0 {
+		t.Fatal("asura Gap = 0, want the Lane's pace")
+	}
+	if asura.Browser || asura.Refusing {
+		t.Fatalf("asura = %+v, want a TLS Lane that is not refusing", asura)
+	}
+	if !kagane.Browser || !kagane.Refusing {
+		t.Fatalf("kagane = %+v, want a browser Lane in refusal backoff", kagane)
+	}
+	if !st.BrowserConfigured || !st.BrowserReachable {
+		t.Fatalf("browser after round = configured=%v reachable=%v, want true/true (sidecar never lost)", st.BrowserConfigured, st.BrowserReachable)
+	}
+
+	// A lost sidecar reads as unreachable for the same window the Lanes skip.
+	p.setBrowserDown(now)
+	if st := p.LaneStatus(); !st.BrowserConfigured || st.BrowserReachable {
+		t.Fatalf("browser after loss = configured=%v reachable=%v, want true/false", st.BrowserConfigured, st.BrowserReachable)
+	}
+}