S3 Plugin Improvements
- S3 removed from backup plan plugin selection — S3 sync is now managed at the repository level only, no longer appears in plan create/edit forms
- Graceful delete protection — deleting an S3 config that's linked to a repository now shows a friendly error instead of a fatal database constraint error
- Global settings validation — warns on save and shows a persistent banner when an S3 config uses global credentials that haven't been configured yet, with a link to Settings