docs: add work log for 2026-06-15 and update DB deletion policy in README

scratch/check_backup_excel.cjs

@@ -0,0 +1,11 @@
+const XLSX = require('xlsx');
+const workbook = XLSX.readFile('backupDB_20260602.xlsx');
+console.log('Sheet Names:', workbook.SheetNames);
+if (workbook.SheetNames.includes('system_users')) {
+  const sheet = workbook.Sheets['system_users'];
+  const data = XLSX.utils.sheet_to_json(sheet);
+  console.log('system_users found! Count:', data.length);
+  console.log('Sample:', data.slice(0, 2));
+} else {
+  console.log('system_users sheet not found in backupDB_20260602.xlsx');
+}