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

.gitea/workflows/ci.yaml

@@ -41,6 +41,27 @@ jobs:
             nuget-${{ runner.os }}-
       - run: make lint
 
+  # The Helm chart's only automated gate: it renders and schema-checks the whole
+  # stack, and checks it still describes the same stack as the compose file
+  # (ADR-0033). No cluster involved — see docs/runbooks/kubernetes-talos.md.
+  k8s:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: https://github.com/actions/checkout@v4
+      # helm as its pinned static binary rather than a marketplace action: one URL,
+      # the same one the Talos runbook §0 gives a developer, and no third-party
+      # action to vet (CLAUDE.md §13). The drift check also needs `docker compose`,
+      # which the runner already has (see docs/runbooks/ci.md).
+      - name: Install helm
+        run: |
+          mkdir -p "$HOME/.local/bin"
+          curl -sSL https://get.helm.sh/helm-v3.16.4-linux-amd64.tar.gz \
+            | tar xz -O linux-amd64/helm > "$HOME/.local/bin/helm"
+          chmod +x "$HOME/.local/bin/helm"
+          echo "$HOME/.local/bin" >> "$GITHUB_PATH"
+      - run: make k8s-lint
+      - run: make k8s-drift
+
   build:
     runs-on: ubuntu-latest
     steps: