Add Organizations::MarkForDeletionService for soft-deletion

What does this MR do and why?

Introduces Organizations::MarkForDeletionService to handle organization soft-deletion by transitioning the organization to the deletion_scheduled state.

The service:

• Checks authorization (delete_organization ability)
• Rejects the default organization
• Rejects non-empty organizations (those with groups or projects)
• Calls schedule_deletion! to transition the state
• Returns ServiceResponse.success with the organization on success
• Logs the event

The EE extension (EE::Organizations::MarkForDeletionService) overrides log_event to additionally emit an audit event (organization_deletion_scheduled).

References

Implements Add Organizations::MarkForDeletionService for s... (#594308 - closed)

Screenshots or screen recordings

N/A — backend service, no UI changes.

Before	After
N/A	N/A

How to set up and validate locally

1. In a Rails console, create a non-default, empty organization and an owner user:

   org = Organizations::Organization.create!(name: 'Test Org', path: 'test-org', visibility_level: 20)
   user = User.first
   org.organization_users.create!(user: user, access_level: :owner)

2. Run the service:

   result = Organizations::MarkForDeletionService.new(org, current_user: user).execute
   result.success? # => true
   org.reload.deletion_scheduled? # => true

MR acceptance checklist

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