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/go.mod

@@ -14,11 +14,16 @@ module fleet-manager
 go 1.25.0
 
 require (
+	github.com/influxdata/influxdb-client-go/v2 v2.14.0
 	google.golang.org/grpc v1.82.0
 	google.golang.org/protobuf v1.36.11
 )
 
 require (
+	github.com/apapsch/go-jsonmerge/v2 v2.0.0 // indirect
+	github.com/google/uuid v1.6.0 // indirect
+	github.com/influxdata/line-protocol v0.0.0-20200327222509-2487e7298839 // indirect
+	github.com/oapi-codegen/runtime v1.0.0 // indirect
 	golang.org/x/net v0.53.0 // indirect
 	golang.org/x/sys v0.43.0 // indirect
 	golang.org/x/text v0.36.0 // indirect