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/secrets/secrets.example.yaml

@@ -1,5 +1,6 @@
-# TEMPLATE ONLY — not part of kustomization.yaml. Copy it to k8s/secrets.yaml (gitignored),
-# fill in real values there, and apply that file yourself. Never put real values in here.
+# TEMPLATE ONLY — not part of kustomization.yaml. Copy it to k8s/secrets/secrets.yaml
+# (gitignored), fill in real values there, and apply that file yourself. Never put real values
+# in here. This folder is outside `kubectl apply -f k8s/`, which does not recurse.
 #
 #   openssl rand -hex 32     for every token and password below
 apiVersion: v1