refactor(external-dns): placement matches neither package category — make it a tenant enabler or move to apps #3275

packages/extra/external-dns fits neither package category as defined:

• It is not a tenant module: there is no toggle in the Tenant chart (packages/apps/tenant/values.yaml only has etcd/monitoring/ingress/gateway/seaweedfs), and its release definition (packages/system/external-dns-rd) is category: Networking with no internal.cozystack.io/tenantmodule label.
• At the same time it is a directly orderable application in the dashboard catalog (kind ExternalDNS), so in practice it behaves like an apps service that happens to live in packages/extra.

Decision rule to apply:

• If the application is a per-tenant singleton → rework it as a proper enabler module: a Tenant chart toggle plus the tenantmodule label, managed under Administration → Modules.
• If it makes sense to order several instances per tenant (e.g. different DNS providers or zones — the chart already ships annotationPrefix, described as "useful for running multiple instances") → move it to packages/apps.

Context: surfaced in the developer guide update (cozystack/website#594) — the new apps-vs-extra decision guide cannot cite external-dns as an example of either category.