Accept x402 payments

The paid endpoint /api/public/x402/demo remains unavailable until configured. The facilitator must explicitly advertise exact USDG on eip155:4663. Protocol compatibility alone is insufficient.

• X402_PAY_TO: receiving EVM address
• X402_FACILITATOR_URL: HTTPS facilitator supporting chain and asset
• X402_PRICE_USDG_BASE_UNITS: positive integer, 6 decimals
• ROBINHOOD_RPC_URL: optional private dedicated mainnet endpoint in Project Settings → Secrets