Fix: Cronjob handling when spider is deleted

[erick-GeGe]

Summary

• Cronjobs are now automatically disabled when their spider is removed in a deploy. A Django signal on Spider.post_save detects the deleted=True transition and disables all active cronjobs for that spider (both SpiderCronJob.status and PeriodicTask.enabled).
• Orphaned cronjobs (whose spider was deleted) can now be managed from the UI — disabled, edited, or deleted. Previously the API returned 404 for any action on them.
• Attempting to re-enable a cronjob whose spider no longer exists now returns a clear error message instead of silently failing.
• Historical spider jobs from deleted spiders are now visible in the cronjob detail page.
• Fixed a silent failure in disable_cronjob / enable_cronjob where an inconsistent PeriodicTask state would cause the operation to silently do nothing.

Issue

• Github Issue ID.

Checklist before requesting a review

• I have performed a self-review of my code.
• My code follows the style guidelines of this project.
• I have made corresponding changes to the documentation.
• New and existing tests pass locally with my changes.
• If this change is a core feature, I have added thorough tests.
• If this change affects or depends on the behavior of other estela repositories, I have created pull requests with the relevant changes in the affected repositories. Please, refer to our official documentation.
• I understand that my pull request may be closed if it becomes obvious or I did not perform all of the steps above.