Legacy cleanup follow-ups after removing WHMCS import and cPanel/WHM (614405ee)

Imported from GitHub issue El-SaMa/oma#141 by @El-SaMa.

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Commit 614405ee removed the one-off WHMCS import tooling and the old cPanel/WHM integration. Imported data and its readers were kept on purpose (see the commit message). Remaining tidy-ups:

1. Unused translation keys from the removed screens: cPanel integration, WHMCS settings, websites.scanCpanel*, ddnsSyncCpanel, ddnsLinkedCpanel and similar. Remove them from all locales.
2. Settings rows / tables no longer read: stored whmcs-mysql credentials in settings_credentials, cPanel/WHM credential storage. Decide whether to delete the rows. Never edit applied migrations; use a new one if dropping tables. Keep cpanel_provisioned_service (history for migrated services).
3. Two legacy cPanel services stay suspended as history: Puunkaatopalvelu O&L Oy and Kaverix (their sites now run on the hosting panel). Operator to decide: terminate them, or keep them as read-only history.
4. Issue #103 mentions cPanel management actions; cPanel is gone, so only Hetzner/Proxmox remain relevant there.
5. Diagrams (apps/web/content/admin-docs/diagrams/platform-overview.drawio, docs/architecture/oma-panel-architecture-simple.drawio) still show the WHMCS ETL box.

Done when

No dead keys or credentials remain, the operator has decided on the 2 legacy services, and the diagrams are updated.