feat: route nofxos data requests through claw402 x402 payment

When CLAW402_WALLET_KEY env var is set, all nofxos.ai data API calls
(AI500, OI rankings, NetFlow, price rankings, etc.) are automatically
routed through claw402.ai with x402 USDC payment instead of direct
free access.

- New Claw402DataClient for GET requests with x402 payment flow
- Endpoint mapping: /api/xxx -> /api/v1/nofx/xxx
- MakeClaw402SignFunc helper for reusable payment signing
- Auto-detection: if CLAW402_WALLET_KEY is set, claw402 mode activates
- Fallback: without wallet key, direct nofxos.ai access (backward compatible)

Env vars:
  CLAW402_WALLET_KEY=0x...  # wallet private key for payment
  CLAW402_URL=https://claw402.ai  # optional, defaults to claw402.ai

mcp/payment/x402.go

@@ -82,6 +82,13 @@ func X402DecodeHeader(b64 string) ([]byte, error) {
 	return decoded, nil
 }
 
+// MakeClaw402SignFunc creates an X402SignFunc from a private key for claw402 payments.
+func MakeClaw402SignFunc(privateKey *ecdsa.PrivateKey) X402SignFunc {
+	return func(paymentHeaderB64 string) (string, error) {
+		return SignBasePaymentHeader(privateKey, paymentHeaderB64, "Claw402")
+	}
+}
+
 // SignBasePaymentHeader decodes a base64 x402 header, parses it, and signs with
 // EIP-712 (USDC TransferWithAuthorization).
 func SignBasePaymentHeader(privateKey *ecdsa.PrivateKey, paymentHeaderB64 string, providerName string) (string, error) {