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>

src/Signals/StellaOps.Signals/Options/SignalsRouterEventsOptions.cs

@@ -8,7 +8,7 @@ public sealed class SignalsRouterEventsOptions
     /// <summary>
     /// Base URL for the StellaOps Router gateway (HTTP ingress).
     /// </summary>
-    public string BaseUrl { get; set; } = "https://gateway.stella-ops.local";
+    public string BaseUrl { get; set; } = "https://router.stella-ops.local";
 
     /// <summary>
     /// Relative path that receives fact update envelopes.