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.

serving/fleet-manager/agent.go

@@ -222,16 +222,17 @@ func desiredMsg(d Desired) *fleetpb.ManagerDown {
 
 func snapshotToHealth(s *fleetpb.HealthSnapshot) Health {
 	return Health{
-		LoadAvg:      s.GetLoadAvg(),
-		CPUPct:       s.GetCpuPct(),
-		TempC:        s.GetTempC(),
-		MemUsedPct:   s.GetMemUsedPct(),
-		TmpfsUsedPct: s.GetTmpfsUsedPct(),
-		NVMeRoot:     s.GetNvmeRoot(),
-		Auth:         s.GetAuth(),
-		SELinux:      s.GetSelinux(),
-		Chrony:       s.GetChrony(),
-		KioskActive:  s.GetKioskActive(),
+		LoadAvg:       s.GetLoadAvg(),
+		CPUPct:        s.GetCpuPct(),
+		TempC:         s.GetTempC(),
+		MemUsedPct:    s.GetMemUsedPct(),
+		TmpfsUsedPct:  s.GetTmpfsUsedPct(),
+		NVMeRoot:      s.GetNvmeRoot(),
+		Auth:          s.GetAuth(),
+		SELinux:       s.GetSelinux(),
+		Chrony:        s.GetChrony(),
+		KioskActive:   s.GetKioskActive(),
+		UptimeSeconds: s.GetUptimeSeconds(),
 	}
 }