Return every error as {"error": "..."}

Echo errors, middleware and handlers used three different formats (message object, error object, bare string). One error handler now writes all of them the same way; status codes are unchanged.

internal/server/statistics_handler.go

@@ -43,7 +43,7 @@ func parseLimit(ctx *echo.Context) (int32, bool) {
 func withLimit[T any](ctx *echo.Context, name string, query func(*echo.Context, int32) (T, error)) error {
 	limit, ok := parseLimit(ctx)
 	if !ok {
-		return ctx.JSON(http.StatusBadRequest, map[string]string{"error": "Invalid limit parameter"})
+		return echo.NewHTTPError(http.StatusBadRequest, "Invalid limit parameter")
 	}
 	result, err := query(ctx, limit)
 	return statsResult(ctx, name, result, err)
@@ -52,7 +52,7 @@ func withLimit[T any](ctx *echo.Context, name string, query func(*echo.Context,
 func statsResult[T any](ctx *echo.Context, name string, result T, err error) error {
 	if err != nil {
 		logging.GetLogger().Error("Failed to get statistics", zap.String("statistic", name), zap.Error(err))
-		return ctx.JSON(http.StatusInternalServerError, map[string]string{"error": "Failed to get statistics"})
+		return echo.NewHTTPError(http.StatusInternalServerError, "Failed to get statistics")
 	}
 	return ctx.JSON(http.StatusOK, result)
 }