refactor backend API structure

internal/httpapi/claude/handler_errors.go

@@ -0,0 +1,25 @@
+package claude
+
+import "net/http"
+
+func writeClaudeError(w http.ResponseWriter, status int, message string) {
+	code := "invalid_request"
+	switch status {
+	case http.StatusUnauthorized:
+		code = "authentication_failed"
+	case http.StatusTooManyRequests:
+		code = "rate_limit_exceeded"
+	case http.StatusNotFound:
+		code = "not_found"
+	case http.StatusInternalServerError:
+		code = "internal_error"
+	}
+	writeJSON(w, status, map[string]any{
+		"error": map[string]any{
+			"type":    "invalid_request_error",
+			"message": message,
+			"code":    code,
+			"param":   nil,
+		},
+	})
+}