feat(tenants): hard-delete (purge) for soft-deleted tenants

Soft-delete kept the slug occupied forever — no way to remove a test tenant
and reuse its name, and external resources lingered. DELETE
/tenants/:slug/purge (platform-admin only, two-step: refuses anything not
already soft-deleted) tears down the Stalwart service + customer domains
(never the platform apex — the management admin account lives there) and
the Authentik group, then removes domains/subscriptions/invoices/user
links/the tenant doc. Audit trail is kept. Operator detail page shows a
'Purge permanently' card once a tenant is soft-deleted.

apps/operator/server/api/tenants/[slug]/purge.delete.ts

@@ -0,0 +1,6 @@
+import { platformApi } from '~~/server/utils/platform-api'
+
+export default defineEventHandler(async (event) => {
+  const slug = getRouterParam(event, 'slug')
+  return platformApi(event, `/tenants/${slug}/purge`, { method: 'DELETE' })
+})