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_test.go

@@ -56,6 +56,18 @@ func TestParseMemUsedPct(t *testing.T) {
 	}
 }
 
+func TestParseUptime(t *testing.T) {
+	if got := parseUptime([]byte("12345.67 98765.43\n")); got != 12345 {
+		t.Fatalf("uptime = %d, want 12345", got)
+	}
+	if got := parseUptime([]byte("")); got != 0 {
+		t.Fatalf("empty uptime = %d, want 0", got)
+	}
+	if got := parseUptime([]byte("garbage")); got != 0 {
+		t.Fatalf("garbage uptime = %d, want 0", got)
+	}
+}
+
 func TestParseHandle(t *testing.T) {
 	for _, tc := range []struct {
 		in   string