Add virtual registries cleanup policies model and DB table

What does this MR do and why?

This MR adds the database table, model for virtual registries clean-up policies. It includes:

1. A new virtual_registries_cleanup_policies database table to store clean-up policy configurations for virtual registries
2. A corresponding VirtualRegistries::Cleanup::Policy model with validations and associations
3. JSON schema validation for detailed clean-up metrics
4. Factory definitions and specs for testing

This implementation is necessary to support automated clean-up of unused artifacts in virtual registries, helping organizations manage storage more efficiently and reduce costs. The clean-up policies will allow administrators to configure retention rules based on time since last download, with configurable cadence for clean-up jobs.

References

• Virtual Registry: cache entry lifecycle last_do... (#548566) • Bonnie Tsang • 18.6

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Run database migrations to create the new table:

   bundle exec rake db:migrate

2. Create a test clean-up policy via rails console:

   group = Group.top_level.first
   policy = VirtualRegistries::Cleanup::Policy.create!(
     group: group,
     enabled: true,
     keep_n_days_after_download: 30,
     cadence: 7,
     notify_on_success: true
   )

3. Test validation rules by attempting invalid configurations:

   invalid_policy = VirtualRegistries::Cleanup::Policy.new(
     group: group,
     keep_n_days_after_download: -1  # Should fail validation
   )
   invalid_policy.valid?  # Should return false
   invalid_policy.errors.full_messages  # Should show validation errors

MR acceptance checklist

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Related to #567200 (closed)