feat(k8s): publish the portals through the labs Caddy (refs #177) (#179)

## What & why

Makes the portals reachable on real hostnames through the Caddy that already fronts `*.labs.respellion.tech`, instead of five SSH port-forwards:

| URL | Service |
|---|---|
| `https://big-register.labs.respellion.tech` | openbaar |
| `https://big-mijn.labs.respellion.tech` | self-service |
| `https://big-behandel.labs.respellion.tech` | behandel |
| `https://big-beheer.labs.respellion.tech` | beheer |
| `https://big-auth.labs.respellion.tech` | Keycloak (`/admin` blocked) |

Chain: browser → labs Caddy (TLS) → `openssh-server` container → reverse SSH tunnel → Fedora host → Talos NodePorts. The Caddy routes and the tunnel unit are already on `main` in the Infra repo (`infra/development/`).

This repo's part:
- **Chart:** a `keycloakUrl` value. When set it replaces `host` + Keycloak's NodePort as the pinned issuer (`KC_HOSTNAME`) and the portals' OIDC authority. Both now come from one helper, `big.keycloakUrl`, so they can't drift apart (ADR-0010). Empty = rendered output identical to today.
- **Deploy workflow:** passes the `KEYCLOAK_URL` repo variable as `--set keycloakUrl=…`.
- **Runbook:** new section "Publishing through the labs Caddy".

Refs #177

## Definition of Done

- [x] Linked Gitea issue (above).
- [ ] Failing test committed before the implementation. *(Infra/config change, no test added.)*
- [x] Implementation makes the test pass; refactor commit if structure improved.
- [x] Conventional Commits referencing the issue (`refs #NN`).
- [ ] CI green — all Gitea Actions jobs (or `make ci` green while no runner exists).
- [x] `docker compose up` from a fresh clone reaches green health checks within 3 minutes. *(Compose untouched.)*
- [x] Docs updated if behaviour, contracts, or operations changed.
- [ ] ADR added in `docs/architecture/` if a non-obvious decision was made.
- [ ] Demo note in `docs/demo-script.md` if user-visible.

## Notes for reviewers

- **This takes the option #177 rejects.** #177 proposes an in-cluster Caddy edge (branch `feat/177-public-tls-edge`). This PR uses the existing labs Caddy instead, because it already holds 80/443 and the wildcard certificate. So it only *refs* #177. If we go this way, #177's ADR should record the host-Caddy option instead.
- `make k8s-lint` and `infra/check-docs-nav.py` pass. I rendered the chart with and without `keycloakUrl`: empty gives the same output as before; set, it gives `https://big-auth.labs.respellion.tech` for both the issuer and the authority.
- Once `KEYCLOAK_URL` is set, the `localhost` port-forward workflow (runbook §5) no longer logs in, because the issuer is a single string.
- The portals are public, with no Azure `authorize` in front of them the way `marketing` has one. The test users use `test123`.
- Rollout after merge: install `big-portals-tunnel.service` on the Fedora host, run `docker compose up -d caddy` on the labs server, then set the `KEYCLOAK_URL` variable.

🤖 Generated with [Claude Code](https://claude.com/claude-code)Reviewed-on: #179

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