ci(k8s): gate the Helm chart in CI + a compose↔chart drift check (closes #168) (#171)

## What & why

The Helm chart landed in #167 with two gaps written into ADR-0033: `make k8s-lint` existed
but no CI job ran it, and *"a second deployment description to keep in step with compose —
nothing enforces that today; a drift check belongs in CI (follow-up)"*. Both are closed here.

**`make k8s-drift`** (`infra/helm/check-drift.py`, stdlib only) compares what each stack
actually deploys rather than diffing two files that differ by design: workload names and
resolved container images, taken from `docker compose config --format json` and a rendered
chart. The six differences that exist today are declared in `DEVIATIONS` with the reason
each was forced — the four `*-init` Django services folded into their web pods, and the two
bootstrap Jobs compose runs from the host — so only a *new* difference fails.

**A `k8s` CI job** runs `k8s-lint` then `k8s-drift` on every push and PR. No cluster, no
marketplace action: helm is fetched as the pinned static binary the Talos runbook already
gives developers.

Closes #168

## Definition of Done

- [x] Linked Gitea issue (above).
- [x] Failing test committed before the implementation — the red commit reports all six
      real differences; the green commit declares them.
- [x] Implementation makes the test pass.
- [x] Conventional Commits referencing the issue (`refs #168`).
- [x] CI green — awaiting the run on this PR (`make k8s-lint` and `make k8s-drift` pass locally).
- [x] `docker compose up` unaffected — no service, image or compose file is touched.
- [x] Docs updated — `docs/runbooks/ci.md` (job table + the one place local and CI now
      differ), `docs/runbooks/kubernetes-talos.md` §7/§"not ported", and ADR-0033's cost note.
- [x] No ADR needed: no new dependency (python stdlib, and helm/docker were already
      prerequisites of the `k8s-*` targets), no boundary moved, no §8 rule bent.
- [x] Not user-visible, so no demo note.

## Notes for reviewers

Verified by hand that both drift classes fail the check, not just that it passes today:

- bumping `OPENZAAK_TAG` in compose alone → reports `openzaak` and `oz-celery` with both
  image strings;
- adding a workload to `values.yaml` alone → reports it by name.

Deliberate limits (there is a `ponytail:` note in the script):

- **Names and images only**, as sets — no per-workload env, ports or volumes. Those differ
  by design in four documented places, so comparing them would mean re-encoding every
  deviation field by field for very little more signal.
- **The three observability workloads are rendered with `enabled=true`** by the check, even
  though both stacks default them off, so their images can't drift unwatched.
- **`k8s-lint`/`k8s-drift` are not in `make ci`**, to avoid making `helm` a hard
  prerequisite for everyone. That is now the only local/CI difference; it's called out in
  `docs/runbooks/ci.md`.

Follow-ups filed while reviewing the chart, not addressed here: #169 (the published docs
omit every ADR after 0010 and all runbooks but `ci.md`) and #170 (the production-posture
ADR #25 asked for — secrets are still plain text in `values.yaml`).Reviewed-on: #171

docs/runbooks/ci.md

@@ -2,8 +2,10 @@
 
 > **Status: active.** The workflow `.gitea/workflows/ci.yaml` runs on Gitea's
 > hosted `ubuntu-latest` runner — no self-hosted runner required.
-> **`make ci` is still the local gate** — it runs the exact same checks
-> (the workflow calls the same `make` targets).
+> **`make ci` is still the local gate** — it runs the same checks via the same
+> `make` targets, with one exception: the `k8s` job's targets are not in `make ci`,
+> because `helm` is optional for everyone not deploying to Kubernetes. Run
+> `make k8s-lint k8s-drift` by hand after touching the chart or the compose file.
 
 ## The pipeline
 
@@ -16,6 +18,8 @@ and CI cannot drift:
 | `lint` | `make lint` → `dotnet format … --verify-no-changes` | .NET 10 SDK |
 | `build` | `make build` → `dotnet build … -c Release` | .NET 10 SDK |
 | `unit` | `make unit` → `dotnet test … -c Release --filter "Category!=Integration"` | .NET 10 SDK |
+| `frontend` | `make frontend` → Nx lint/test/build for the four portals | pnpm + Node |
+| `k8s` | `make k8s-lint` (render + schema-check the Helm chart) → `make k8s-drift` (chart still describes the same stack as `infra/docker-compose.yml`) | pinned `helm` binary + `docker compose` |
 | `mutation` | `make mutation` → `dotnet tool restore` → `dotnet stryker` (ACL); uploads the HTML report as an artifact | .NET 10 SDK |
 | `verify-stack` | the single live-stack stage — steps: `make verify-up` (full stack up + health, the DoD smoke) → `make verify-acl` (ACL ↔ OpenZaak) → `make verify-nrc` (OpenZaak → NRC delivery) → `make down` | container engine + egress (base images, nuget, `selectielijst.openzaak.nl`) |
 

docs/runbooks/kubernetes-talos.md

@@ -322,6 +322,7 @@ The PVCs carry `helm.sh/resource-policy: keep`, so `make k8s-down` leaves the da
 
 ```bash
 make k8s-lint                      # render + schema-check the chart, no cluster needed
+make k8s-drift                     # fail if compose and the chart describe different stacks
 make k8s-portals                   # forward the portals + Keycloak to localhost (browser access)
 make k8s-images K8S_REGISTRY=...   # after changing a service or a portal
 make k8s-up TALOS_HOST=... K8S_REGISTRY=...
@@ -366,5 +367,7 @@ immutable, so `helm upgrade` is rejected with `cannot patch "…" with kind Job`
   `K8S_SET='--set workloads.tempo.enabled=true --set workloads.prometheus.enabled=true --set workloads.grafana.enabled=true'`.
   The .NET services still export OTLP; the exporter fails harmlessly when Tempo is absent.
 - **The verify/e2e lanes.** `make verify*` and the Playwright e2e drive compose, not the
-  chart. The Kubernetes path is verified with §5's smoke test.
+  chart. The Kubernetes path is verified with §5's smoke test. CI's `k8s` job runs the two
+  clusterless checks (`k8s-lint`, `k8s-drift`) on every PR — a values typo or a compose
+  image bump that skipped the chart fails there, but nothing deploys the chart in CI.
 - **Ingress, TLS, and resource requests.** See the ponytail ceiling in ADR-0033.