Fix cron validation to support */N step notation

Updated regex to properly support step notation with asterisk (e.g., */2, */5).

Now supports all common cron patterns:
- * (wildcard)
- */2 (every 2 units - step notation)
- 5 (specific value)
- 1-5 (range)
- 1-5/2 (step within range)
- 1,3,5 (list)
- 10-20/5 (step within range)

Changed regex from:
  /^(\*|(\d+(-\d+)?(,\d+(-\d+)?)*)(\/\d+)?)$/
To:
  /^(\*\/\d+|\*|\d+(-\d+)?(\/\d+)?(,\d+(-\d+)?(\/\d+)?)*)$/

The key change is adding \*\/\d+ as the first alternative to explicitly
match step notation like */2, */5, */10, etc.

Backend already supports this via Django Celery Beat's CrontabSchedule,
which accepts standard cron syntax including step notation.

frontend/src/components/backups/BackupManager.jsx

@@ -151,8 +151,9 @@ function validateCronExpression(expression) {
 
   const [minute, hour, dayOfMonth, month, dayOfWeek] = parts;
 
-  // Validate each part (allowing *, ranges, lists, steps)
-  const cronPartRegex = /^(\*|(\d+(-\d+)?(,\d+(-\d+)?)*)(\/\d+)?)$/;
+  // Validate each part (allowing *, */N steps, ranges, lists, steps)
+  // Supports: *, */2, 5, 1-5, 1-5/2, 1,3,5, etc.
+  const cronPartRegex = /^(\*\/\d+|\*|\d+(-\d+)?(\/\d+)?(,\d+(-\d+)?(\/\d+)?)*)$/;
 
   if (!cronPartRegex.test(minute)) {
     return { valid: false, error: 'Invalid minute field (0-59, *, or cron syntax)' };