Make retries safe with idempotency keys

curl -X POST https://api.paxalabs.com/v1/tts \
  -H "Authorization: Bearer $PAXA_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: order-8146-confirmation" \
  -d '{"text": "สวัสดีครับ ยินดีต้อนรับสู่ Paxa Labs", "voice": "khanomkrok", "model": "paxa-tts-flash-v2"}' \
  --output speech.mp3

// Reuse the same key when retrying the identical request: the charge
// happens once and the same audio is served again.
const idempotencyKey = `order-${order.id}-confirmation`;

const response = await fetch("https://api.paxalabs.com/v1/tts", {
  method: "POST",
  headers: {
    Authorization: `Bearer ${process.env.PAXA_API_KEY}`,
    "Content-Type": "application/json",
    "Idempotency-Key": idempotencyKey,
  },
  body: JSON.stringify({ text: "สวัสดีครับ ยินดีต้อนรับสู่ Paxa Labs", voice: "khanomkrok", model: "paxa-tts-flash-v2" }),
});