[Feat] Support cron-based scheduling for rank tracking

Currently, rank tracking frequency can only be configured using intervals such as daily, weekly, etc.

It would be useful to also specify the exact day and time when the rank tracking should run.

For example:

• Frequency: Weekly
• Day: Sunday
• Time: 9:00 PM

This could potentially be implemented using a cron expression.

For example, running every Sunday at 9:00 PM:

0 21 * * 0

This would make rank tracking schedules more predictable and would also make it easier to coordinate them with other automated workflows that depend on the rank tracking results.