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/proto/fleet_agent.proto

@@ -54,9 +54,11 @@ message HealthSnapshot {
   string selinux         = 9;  // enforcing|permissive
   string chrony          = 10; // synced|unsynced
   bool   kiosk_active    = 11;
-  string session_uptime  = 12;
+  string session_uptime  = 12;  // KIOSK session uptime (advisory string)
   int32  chromium_crashes = 13;
   string display         = 14;
+  int64  uptime_seconds  = 15;  // HOST uptime (seconds since boot) — boot_time =
+                                // manager receive-time - this; drives reboot-pending
 }
 
 message CommandResult {