402 payment loop

1. Call any endpoint with no key. Read X-Credits-Remaining to see how many free requests are left today.
2. When the allowance or a key's credits run out, the API answers 402 with error "keyless_limit" or "insufficient_credits", an X-Payment-URL header and a JSON body with action and packs.
3. Show your human action.prompt_to_user and stop retrying. Every link in it (action.url, packs[].checkout_url) opens Stripe Checkout in one click.
4. After payment the success page shows the key and copies it to the clipboard. Retry with Authorization: Bearer <key>.

HTTP 402 body

{
  "error": "keyless_limit",
  "message": "Daily free tier limit (25 requests) reached. Reload 15,000 searches for $15.",
  "checkout_url": "https://contextfetch.dev/#buy",
  "pricing": {
    "pack_size": 15000,
    "price_usd": 15,
    "unit_cost_usd": 0.001
  },
  "packs": [
    {
      "id": "instant",
      "name": "Instant",
      "credits": 15000,
      "price_usd": 15,
      "cost_per_1k": 1,
      "recommended": false,
      "checkout_url": "https://api.contextfetch.dev/buy/instant"
    },
    {
      "id": "builder",
      "name": "Builder",
      "credits": 60000,
      "price_usd": 50,
      "cost_per_1k": 0.83,
      "recommended": true,
      "checkout_url": "https://api.contextfetch.dev/buy/builder"
    },
    {
      "id": "scale",
      "name": "Scale",
      "credits": 200000,
      "price_usd": 150,
      "cost_per_1k": 0.75,
      "recommended": false,
      "checkout_url": "https://api.contextfetch.dev/buy/scale"
    }
  ],
  "action": {
    "type": "payment_link",
    "url": "https://api.contextfetch.dev/buy/instant",
    "prompt_to_user": "ContextFetch credits ran out. Reload 15,000 searches for $15: https://api.contextfetch.dev/buy/instant"
  }
}

On /tavily/* the same condition returns 432 with {"detail": {"error": "..."}} and the same X-Payment-URL header. checkout_url (https://contextfetch.dev/#buy) stays in the body for older clients.