fix(helm): add configurable namespace override to chart templates

[casinesque]

What

Adds an explicit metadata.namespace to all chart templates, defaulting to
.Release.Namespace, and exposes a namespace value to override it.

Closes #432

Why

I thought this could be useful since some tools (e.g. GitOps controllers using
helm template under the hood) expect resources to declare namespace
explicitly rather than relying on the implicit release namespace.

Change

The fix was quite simple: added namespace: {{ .Values.namespace | default .Release.Namespace }}
to metadata in each template, and added a namespace: "" field in
values.yaml to allow overriding the default.

Testing

• helm template . — renders identically to before (fallback to
  .Release.Namespace, no behavior change)
• helm template . --set namespace=custom-ns — all resources render with
  namespace: custom-ns

Backwards compatibility

Non-breaking: default behavior is unchanged when namespace is not set.