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/inventory.go

@@ -49,16 +49,17 @@ type Version struct {
 // (written by the manager as agent reports arrive); this cached last value is
 // what the fleet-overview list renders without a per-host Flux query.
 type Health struct {
-	LoadAvg      float64 `json:"load_avg,omitempty"`
-	CPUPct       float64 `json:"cpu_pct,omitempty"`
-	TempC        float64 `json:"temp_c,omitempty"`
-	MemUsedPct   float64 `json:"mem_used_pct,omitempty"`
-	TmpfsUsedPct float64 `json:"tmpfs_used_pct,omitempty"` // overlay RAM pressure (ties to #17)
-	NVMeRoot     string  `json:"nvme_root,omitempty"`      // ok|degraded|down
-	Auth         string  `json:"auth,omitempty"`           // 802.1x/macsec: up|down
-	SELinux      string  `json:"selinux,omitempty"`        // enforcing|permissive
-	Chrony       string  `json:"chrony,omitempty"`         // synced|unsynced
-	KioskActive  bool    `json:"kiosk_active"`
+	LoadAvg       float64 `json:"load_avg,omitempty"`
+	CPUPct        float64 `json:"cpu_pct,omitempty"`
+	TempC         float64 `json:"temp_c,omitempty"`
+	MemUsedPct    float64 `json:"mem_used_pct,omitempty"`
+	TmpfsUsedPct  float64 `json:"tmpfs_used_pct,omitempty"` // overlay RAM pressure (ties to #17)
+	NVMeRoot      string  `json:"nvme_root,omitempty"`      // ok|degraded|down
+	Auth          string  `json:"auth,omitempty"`           // 802.1x/macsec: up|down
+	SELinux       string  `json:"selinux,omitempty"`        // enforcing|permissive
+	Chrony        string  `json:"chrony,omitempty"`         // synced|unsynced
+	KioskActive   bool    `json:"kiosk_active"`
+	UptimeSeconds int64   `json:"uptime_seconds,omitempty"` // host uptime at last report; boot_time = last_seen - this (reboot-pending)
 }
 
 // Usage — kiosk-session facts (agent-reported).