feat: restrict backup archive download to primary admin and hide admin-hidden projects from other admins

- Add `get_admin_uids()` and `get_primary_admin_uid()` to database.py for resolving the earliest-created admin
- Modify `can_view_project()` in content.py so a project hidden by an admin is invisible to other admins (both web UI and REST API)
- Update `_download_url()` and `_backup_payload()` in admin/backups.py to accept a `can_download` flag, gating the download endpoint with `is_primary_admin()`
- Remove `role` from `_user_facts()` in docs_live.py to avoid leaking admin status in live docs
- Update doc summaries in docs_api.py to reflect the new admin-visibility and backup-download semantics

devplacepy/static/css/feed.css

@@ -96,7 +96,7 @@
     color: var(--text-primary);
 }
 
-.post-author-role {
+.post-author-level {
     font-size: 0.6875rem;
     color: var(--text-muted);
     font-weight: 400;