fix(kpi): don't number vision/values/uniqueness/identity as strategic issues

Add is_reference_section flag to header_data/detail_kpi_data so the
kpi_performance_result and detail_kpi Excel exports render the 4
reference-indicator sections (วิสัยทัศน์/ค่านิยม/เอกลักษณ์/อัตลักษณ์)
with their bare category name instead of a "ประเด็นยุทธ์ศาสตร์ที่ N"
prefix, and skip them in the real strategic-issue numbering.

Models/budget/kpi_performance_results.cs

@@ -14,6 +14,8 @@ namespace rmutr_report.Models
     {
         public string strategic_issues { get; set; }
 
+        public bool is_reference_section { get; set; }
+
         public List<detail_kpi_performance_result> data { get; set; }
 
     }