List saved payment methods

List the caller's organization's saved Stripe payment methods (cards). Returns up to 5 most recent. Requires a `vr_session_*` token from `voxrouter login`. Returns an empty list for callers with no organization.

curl https://api.voxrouter.ai/v1/billing/methods \
  -H "Authorization: Bearer $VOXROUTER_API_KEY"

{
  "methods": [
    {
      "id": "string",
      "brand": "string",
      "last4": "string",
      "expMonth": 0,
      "expYear": 0
    }
  ]
}