Use shared HTTP client for upstream requests

- Create package-level httpClient with 300s timeout\n- Reuse client instead of creating new one per request\n- Prevents resource exhaustion under load\n- Reduces connection overhead

handler.go

@@ -19,6 +19,9 @@ type Config struct {
 
 var config *Config
 
+// httpClient is a shared HTTP client for all upstream requests
+var httpClient = &http.Client{Timeout: 300 * time.Second}
+
 // blockedHeaders are headers that should never be forwarded to upstream
 // for security/privacy reasons. These headers could leak internal URLs,
 // session information, or other sensitive data.
@@ -218,8 +221,7 @@ func callUpstream(req *AnthropicRequest, apiKey, sessionID string) (*http.Respon
 		httpReq.Header.Set(k, v)
 	}
 
-	client := &http.Client{Timeout: 300 * time.Second}
-	return client.Do(httpReq)
+	return httpClient.Do(httpReq)
 }
 
 func writeError(w http.ResponseWriter, code int, message, errType, errCode string) {