Enqueue an asynchronous SMTP send-test

Enqueues a send-test email via the transactional outbox and returns `204` IMMEDIATELY — there is NO synchronous SMTP in the request path. The scheduler delivers using the tenant's resolved SMTP client and writes the outcome to the `last_test_*` fields surfaced by `GET /v1/admin/settings/smtp`, which the UI polls. Caller must hold `tenant.settings.secrets.write`. Plan 2026-06-16-per-tenant-settings §2.6 / T103.

curl -X POST "https://example.com/v1/admin/settings/smtp/test"

{
  "error": "unauthenticated",
  "message": "missing or invalid bearer token",
  "request_id": "req_01abc123"
}

{
  "error": "permission_denied",
  "message": "caller does not hold the required permission",
  "request_id": "req_01abc124"
}

{
  "error": "not_found",
  "message": "the requested resource was not found",
  "request_id": "req_01abc125"
}