Clean up attachments from incomplete form uploads

[pdclark]

Summary

• Tags file field attachments with _pods_pending_form_upload post meta (timestamp) when uploaded via AJAX in Pods->form()
• Removes the flag when the form is successfully saved via PodsField_File::save()
• Adds a daily WP-Cron job that deletes flagged attachments that are no longer pending
• Skips attachments uploaded within the last 12 hours to allow time for forms still being filled out
• Unschedules the cron on plugin deactivation

Fixes #7459

Test plan

• Create a Pod with a File/Image/Video field and render with $pod->form()
• Upload a file — verify _pods_pending_form_upload meta is set on the attachment
• Submit the form — verify the meta is removed
• Upload a file and abandon the form — verify meta persists
• Trigger cron manually (do_action('pods_cleanup_pending_form_uploads')) with a backdated timestamp — verify the attachment is deleted
• Verify attachments uploaded less than 12 hours ago are not deleted by the cron

[what-the-diff]

PR Summary

• Implemented Daily Cleanup for Unfinished Form Uploads
  We've added a new process that runs every day. This process will find and delete any form attachments that were uploaded but never used because the form wasn't completely filled out.

• Deletion Logic for Aged Pending Attachments
  The daily process specifically locates and removes any attachments marked as "pending" that have been left unused for more than 12 hours. This keeps our system clean and more efficient.

• Scheduled Cleanup Deactivated Along With Plugin
  When the Pods plugin is deactivated, this daily cleanup process is also halted. This ensures unnecessary tasks aren't running when they're not needed.

• Marking Uploads for Future Removal
  Whenever a file is uploaded as a form attachment, it will initially be marked as "pending". Only when the form is successfully completed, this mark will be removed. This strategy enables the daily cleanup process to identify which of the attachments can be removed.