Charge a saved card to top up the wallet

Charge a previously-saved Stripe payment method off-session and credit the caller's organization wallet. Accepts both `vr_session_*` (CLI) and raw better-auth session tokens (dashboard server-side). `pk_*` keys are rejected — minting credit purchases from an API key would be a privilege-escalation footgun. Idempotency is REQUIRED. Pass a per-attempt `idempotency_key` — Stripe uses it to dedupe retries. The SDK generates a fresh UUID per call when callers don't supply one. If the card requires a 3D Secure challenge, the response is `402 stripe_3ds_required` with a URL pointing at the dashboard's PaymentIntents flow. The CLI prints the URL; the user finishes the challenge in a browser, after which the card becomes off-session-friendly for subsequent topups.

curl -X POST https://api.voxrouter.ai/v1/billing/topup \
  -H "Authorization: Bearer $VOXROUTER_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
      "amount_cents": 0,
      "payment_method_id": "string",
      "idempotency_key": "string"
    }'

{
  "ok": false,
  "payment_intent_id": "string",
  "amount_cents": 0
}