refactor backend API structure

internal/httpapi/gemini/handler_errors.go

@@ -0,0 +1,28 @@
+package gemini
+
+import "net/http"
+
+func writeGeminiError(w http.ResponseWriter, status int, message string) {
+	errorStatus := "INVALID_ARGUMENT"
+	switch status {
+	case http.StatusUnauthorized:
+		errorStatus = "UNAUTHENTICATED"
+	case http.StatusForbidden:
+		errorStatus = "PERMISSION_DENIED"
+	case http.StatusTooManyRequests:
+		errorStatus = "RESOURCE_EXHAUSTED"
+	case http.StatusNotFound:
+		errorStatus = "NOT_FOUND"
+	default:
+		if status >= 500 {
+			errorStatus = "INTERNAL"
+		}
+	}
+	writeJSON(w, status, map[string]any{
+		"error": map[string]any{
+			"code":    status,
+			"message": message,
+			"status":  errorStatus,
+		},
+	})
+}