fix: SSRF IPv6 bypass, SIGINT handler, README accuracy fixes

fix: extract payer from Soroban auth entries, synchronous refund in handler

fix: debug logging for payment response header extraction

fix: move refund hook before x402 middleware, use res.on finish for post-settlement payer extraction

fix: add debug logging for payer extraction, fallback to response header

fix: use base64 not base64url for payment header decoding

fix: extract payer from tx XDR, refund on crash, memo length guard, MCP refund info

feat: auto-refund for failed renders (blocked/empty pages) via Stellar USDC transfer

RenderGate: x402-powered headless rendering API on Stellar