Add money for ads

Returns a browser link where the owner adds money for ads (first top-up $100 minimum). Call it only after launch_ads, once the owner agreed a daily budget — never on a vague "ok". BenjiAds itself is free; the ad balance is the only money. This call charges nothing: hand the link over, never collect card details, never say the payment succeeded until get_overview shows the money.

curl -X POST "https://example.com/billing/checkout" \  -H "Idempotency-Key: stringst"

{  "url": "http://example.com",  "action": "complete_in_browser"}