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

provider/nofxos/client.go

@@ -21,10 +21,11 @@ const (
 
 // Client is the NofxOS API client
 type Client struct {
-	BaseURL string
-	AuthKey string
-	Timeout time.Duration
-	mu      sync.RWMutex
+	BaseURL      string
+	AuthKey      string
+	Timeout      time.Duration
+	mu           sync.RWMutex
+	claw402      *Claw402DataClient // If set, routes requests through claw402
 }
 
 var (
@@ -71,6 +72,13 @@ func (c *Client) SetConfig(baseURL, authKey string) {
 	}
 }
 
+// SetClaw402 enables routing requests through claw402 payment gateway.
+func (c *Client) SetClaw402(claw402Client *Claw402DataClient) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.claw402 = claw402Client
+}
+
 // GetBaseURL returns the current base URL
 func (c *Client) GetBaseURL() string {
 	c.mu.RLock()
@@ -85,14 +93,22 @@ func (c *Client) GetAuthKey() string {
 	return c.AuthKey
 }
 
-// doRequest performs an HTTP GET request with authentication
+// doRequest performs an HTTP GET request with authentication.
+// If claw402 client is configured, routes through claw402 payment gateway instead.
 func (c *Client) doRequest(endpoint string) ([]byte, error) {
 	c.mu.RLock()
+	claw402Client := c.claw402
 	baseURL := c.BaseURL
 	authKey := c.AuthKey
 	timeout := c.Timeout
 	c.mu.RUnlock()
 
+	// Route through claw402 if configured
+	if claw402Client != nil {
+		return claw402Client.DoRequest(endpoint)
+	}
+
+	// Direct call to nofxos.ai
 	url := baseURL + endpoint
 	if !strings.Contains(url, "auth=") {
 		if strings.Contains(url, "?") {