feat(k8s): publish the portals through the labs Caddy (refs #177)

Adds a `keycloakUrl` chart value: when set, it replaces host + Keycloak's
NodePort as the pinned issuer and the portals' OIDC authority, so the stack
works behind the existing *.labs.respellion.tech Caddy over HTTPS. Empty keeps
today's behaviour. The deploy workflow passes the KEYCLOAK_URL repo variable.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

infra/helm/big-reference/values.yaml

@@ -25,6 +25,11 @@
 # string, so browser tokens and the BFF's discovered issuer agree.
 host: 192.168.122.100
 
+# Set when a TLS proxy outside the cluster publishes Keycloak: the full origin, no
+# trailing slash. It replaces `host` + Keycloak's NodePort as the issuer and the
+# portals' authority (runbook, "Publishing through the labs Caddy").
+keycloakUrl: ""
+
 # Set when pulling from a private registry (e.g. the Gitea Container Registry).
 imagePullSecrets: []
 
@@ -268,7 +273,7 @@ workloads:
       # Pin the issuer to the address the browser uses, and let backchannel calls
       # keep using keycloak:8080 — the BFF discovers metadata in-cluster and gets
       # this issuer back, which is what browser tokens carry (infra/host-browser.yml).
-      KC_HOSTNAME: "http://{{ .Values.host }}:{{ index .Values.nodePorts \"keycloak\" }}"
+      KC_HOSTNAME: '{{ include "big.keycloakUrl" . }}'
       KC_HOSTNAME_BACKCHANNEL_DYNAMIC: "true"
     ports: [{ name: http, port: 8080 }]
     # TCP, not /health/ready on the management port: nothing here gates on realm