[CLUST-348] remove Slurm accounts when archiving groups

[joegoooo]

Type of changes

• Feature

Purpose

Why: Archived groups currently retain their Slurm accounts, meaning they still have access to compute resources. An archived group should not be able to submit jobs.

What: Delete a group's Slurm account tree when archiving, and recreate it when unarchiving — so archived groups lose compute access and unarchived groups regain it.

How: Added saga steps to the existing Archive and Unarchive methods in internal/group/service.go:

• Archive — after removing LDAP users, deletes the Slurm account tree children-first (admin → base → top), mirroring the existing Delete method's pattern
• Unarchive — after unarchiving the DB record, recreates the Slurm account tree top-first then children, mirroring the existing Create method's pattern

Both use the saga pattern with compensation callbacks for rollback on failure.

Additional Information

• No interface changes (Store, SlurmStore signatures unchanged)
• No handler changes needed
• Slurm account hierarchy: top-level (<group-cn>) → children (<group-cn>-base, <group-cn>-admin)
• Deletion order is children-first because slurmdbd refuses to delete an account with child associations
• The saga compensation logic mirrors what Create and Delete already do, ensuring consistency across all group lifecycle operations