fleet-manager: checkpoint 4 — InfluxDB metrics gateway (write + Flux read)

The manager is the ONLY thing that talks to InfluxDB, both directions; hosts hold
no token and never reach it.

- influx.go: influxSink implements MetricsSink — each agent HealthSnapshot is
  written as a fleet_health point, identity (serial/hostnqn/starship) as TAGS
  from the AUTHENTICATED stream (a host can only write its own series), telemetry
  as fields. QueryField answers the SPA's read: a Flux query built with a strict
  FIELD WHITELIST + hex-serial guard + parsed-duration range, so no caller input
  is interpolated raw (injection-safe). Server-side influxdb-client-go/v2.
- admin.go: GET /api/hosts/{serial}/metrics?field=&range= -> time-series JSON;
  503 when InfluxDB isn't wired, 400 on a bad range, 404 for unknown host.
- main.go: sink created when FLEET_MGR_INFLUX_URL+_TOKEN set, fed to the agent
  hub; nil -> no-op sink so the manager runs without metrics.

Tests: snapshotPoint tags/fields/time, fluxQuery contents + %q quoting, and
QueryField rejects a non-hex serial + non-whitelisted field. build/test/vet/gofmt
clean. Live InfluxDB path validated in the compose bench (ckpt 6).

serving/fleet-manager/main.go

@@ -42,6 +42,9 @@ type config struct {
 	brokerClientKey  string
 	brokerCA         string
 
+	// InfluxDB (the manager is the only reader/writer; token is server-side only).
+	influx influxConfig
+
 	inventoryPath string
 	seed          bool
 	hostnqnBase   string
@@ -65,6 +68,13 @@ func loadConfig() config {
 		brokerClientKey:  env("FLEET_MGR_BROKER_CLIENT_KEY", ""),
 		brokerCA:         env("FLEET_MGR_BROKER_CA", ""),
 
+		influx: influxConfig{
+			url:    env("FLEET_MGR_INFLUX_URL", ""),
+			token:  env("FLEET_MGR_INFLUX_TOKEN", ""),
+			org:    env("FLEET_MGR_INFLUX_ORG", "fleet"),
+			bucket: env("FLEET_MGR_INFLUX_BUCKET", "fleet_health"),
+		},
+
 		inventoryPath: env("FLEET_MGR_INVENTORY", "/var/lib/fleet-manager/inventory.json"),
 		seed:          env("FLEET_MGR_SEED", "") == "true",
 		hostnqnBase:   env("HOSTNQN_BASE", "nqn.2026-06.org.example.fleet:host-"),
@@ -77,6 +87,7 @@ type server struct {
 	store  Store
 	hub    *agentHub     // live agent streams; nil if the agent plane isn't configured
 	broker *brokerClient // enrol-broker admin client; nil if authorise isn't configured
+	influx *influxSink   // metrics gateway; nil if InfluxDB isn't configured
 }
 
 func main() {
@@ -106,11 +117,24 @@ func main() {
 		log.Printf("authorise pillar DISABLED (set FLEET_MGR_BROKER_URL + client cert/key + CA to enable)")
 	}
 
+	// Metrics gateway: the manager is the only reader/writer of InfluxDB. nil (a
+	// no-op MetricsSink) when unconfigured, so the manager runs without metrics.
+	var sink MetricsSink
+	if cfg.influx.url != "" && cfg.influx.token != "" {
+		s.influx = newInfluxSink(cfg.influx)
+		defer s.influx.Close()
+		sink = s.influx
+		log.Printf("InfluxDB metrics gateway -> %s (org=%s bucket=%s)", cfg.influx.url, cfg.influx.org, cfg.influx.bucket)
+	} else {
+		log.Printf("InfluxDB DISABLED (set FLEET_MGR_INFLUX_URL + _TOKEN to enable metrics)")
+	}
+
 	// Agent plane (gRPC/mTLS, fleet CA). Started only when configured; the admin
 	// plane can run standalone (e.g. before any host is enrolled). The hub is the
-	// live-stream registry the lifecycle API pushes desired-state through.
+	// live-stream registry the lifecycle API pushes desired-state through, and it
+	// writes each snapshot to the metrics sink.
 	if cfg.agentCert != "" && cfg.agentKey != "" && cfg.agentClientCA != "" {
-		s.hub = newAgentHub(store, nil) // InfluxDB sink wired in checkpoint 4
+		s.hub = newAgentHub(store, sink)
 		go func() {
 			if err := serveAgent(cfg, s.hub); err != nil {
 				log.Fatalf("agent plane: %v", err)