k8s: work with kubectl apply -f as well as -k

- The secrets template moves to k8s/secrets/, and the filled-in copy to
  k8s/secrets/secrets.yaml (gitignored). `kubectl apply -f k8s/` does
  not recurse, so neither the placeholder template nor the real secrets
  can be applied by accident.
- The server image digest is pinned in server.yaml instead of through
  kustomize's images override, which `-f` ignores (it would have pulled
  a nonexistent :latest).
- README and kustomization.yaml say to use `-k`: plain `-f` never
  creates the generated Grafana ConfigMaps.
- The IngressRoutes drop `tls: {}`, like the other routes on this
  Traefik: TLS is configured on the websecure entrypoint.

Manifests validated both ways (14 resources rendered with kustomize,
11 as plain files — the difference is the Grafana ConfigMaps).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QLJfZQZeAKAwSqViTmLzki

k8s/server.yaml

@@ -29,7 +29,8 @@ spec:
           type: RuntimeDefault
       containers:
         - name: server
-          image: git.stinnesbeck.com/nils/mg4-server:latest # pinned by kustomization.yaml
+          # 2.1.0, pinned by digest so a re-pushed tag cannot change what runs.
+          image: git.stinnesbeck.com/nils/mg4-server@sha256:e5f6e2d2ef9dc2650055a0823a3f86d8903e46918a76e990540482edf819766d
           ports:
             - name: http
               containerPort: 8080