fleet-manager: host uptime metric + reboot-pending status

The manager lets an admin schedule a reboot (desired.reboot_at) but never showed
whether it had happened. Add host uptime to the health snapshot and derive a
reboot status from it.

- proto: HealthSnapshot.uptime_seconds (15); regenerated pb.go (manager + agent copy).
- agent: probe /proc/uptime -> uptime_seconds (with parser + test).
- manager: Health.UptimeSeconds cached in inventory; snapshotToHealth maps it;
  influx writes + whitelists uptime_seconds as a queryable field.
- UI: fmt.js dur() + rebootStatus() — boot_time = last_seen - uptime (manager clock
  frame, no host/manager sync needed), compared to reboot_at. Monitor host detail
  shows an "uptime" row and a "reboot" row: pending in X / overdue by X / rebooted.

Observability-only — works before the reboot ACTUATOR lands (deferred). Once a host
boots after the scheduled time (BMC/manual now, agent later), the badge flips to
"rebooted" automatically.

files/agent/snapshot.go

@@ -98,10 +98,34 @@ func (p *prober) snapshot(ctx context.Context) *fleetpb.HealthSnapshot {
 		Chrony:        chronyState(),
 		KioskActive:   unitActive("kiosk.service"),
 		SessionUptime: f.Kiosk.Uptime,
+		UptimeSeconds: uptimeSeconds(),
 	}
 	return s
 }
 
+// uptimeSeconds returns host uptime (seconds since boot) from /proc/uptime. The
+// manager derives boot_time = receive-time - this to tell whether a scheduled
+// reboot has been actioned yet.
+func uptimeSeconds() int64 {
+	b, err := os.ReadFile("/proc/uptime")
+	if err != nil {
+		return 0
+	}
+	return parseUptime(b)
+}
+
+func parseUptime(b []byte) int64 {
+	fields := strings.Fields(string(b))
+	if len(fields) == 0 {
+		return 0
+	}
+	f, _ := strconv.ParseFloat(fields[0], 64)
+	if f < 0 {
+		return 0
+	}
+	return int64(f)
+}
+
 // ---- individual probes (all best-effort) ----------------------------------
 
 func loadAvg() float64 {