Eliminate legacy gateway container (consolidate into router-gateway)

The gateway service was a redundant deployment of the same
StellaOps.Gateway.WebService binary already running as router-gateway.
It served no unique purpose — all traffic is handled by router-gateway
(slot 0). This removes the container, its route table entries, nginx
proxy blocks, health/quota stubs, and redirects STELLAOPS_GATEWAY_URL
to router.stella-ops.local so the Angular frontend resolves API base
URLs through the canonical frontdoor.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/modules/router/rollout-acceptance-20260222.md

@@ -10,7 +10,7 @@
 - Image refresh:
 - `docker build ... -t stellaops/scanner-web:dev`
 - `docker build ... -t stellaops/integrations-web:dev`
-- `docker build ... -t stellaops/gateway:dev`
+- _(stellaops/gateway:dev removed — consolidated into router-gateway)_
 - `docker build ... -t stellaops/doctor-web:dev`
 - Clean bootstrap:
 - `devops/compose/scripts/router-mode-redeploy.ps1 -Mode reverseproxy`