add POST /logs installer log relay to VictoriaLogs

A host being PXE-discovered or installed is not in Kubernetes, so vlagent
cannot collect its logs and a failed install leaves no record; the installer
environment has no internal-CA trust or credentials for the HTTPS log ingest,
and bootapi is already the plain-HTTP broker it can reach.

- add POST /logs, token-guarded like POST /provisioned, relaying ndjson to
  vlinsert's jsonline endpoint keyed on serial+phase
- stamp observed source IP and resolved NetBox device name into extra_fields
- return 202 on a sink failure so logs never block an install
- add BOOTAPI_VLINSERT_URL/_TIMEOUT and bootapi_log_relay metrics

cmd/bootapi/main.go

@@ -47,7 +47,10 @@ func main() {
 		slog.Warn("no NetBox token set (BOOTAPI_NETBOX_TOKEN/_FILE); NetBox reads will likely be denied")
 	}
 	if cfg.ProvisionToken == "" {
-		slog.Warn("no BOOTAPI_PROVISION_TOKEN set; the /provisioned callback is disabled (pxe_enabled will not auto-clear)")
+		slog.Warn("no BOOTAPI_PROVISION_TOKEN set; the /provisioned callback and /logs relay are disabled")
+	}
+	if cfg.VLInsertURL == "" {
+		slog.Warn("BOOTAPI_VLINSERT_URL is empty; the /logs installer log relay is disabled")
 	}
 
 	rcfg := render.RenderConfig{
@@ -92,6 +95,8 @@ func main() {
 		Cache:              cache,
 		UnknownMACFallback: cfg.UnknownMACFallback,
 		ProvisionToken:     cfg.ProvisionToken,
+		VLInsertURL:        cfg.VLInsertURL,
+		VLInsertTimeout:    cfg.VLInsertTimeout,
 		TLSAddr:            cfg.TLSListenAddr,
 		TLSCertFile:        cfg.TLSCertFile,
 		TLSKeyFile:         cfg.TLSKeyFile,