Stable v2.8.9

Backup prune always finishes now — even on slow FTP. The background retention prune's pack-repacking is now bounded per run (--max-repack-size), so it completes reliably and reclaims space incrementally across weekly runs instead of grinding for hours on a slow FTP target (fully-unreferenced data is still freed in full immediately). FTP repos get a tight 64M default; S3/local stay unbounded. Tunable per install via BACKUP_PRUNE_MAX_REPACK.

• fix(backup): bound the periodic prune's repack so it always finishes (esp. FTP)
• docs(api): note that the interactive API docs at /api/docs require ENABLE_API_DOCS=true (off by default for security)
• docs(changelog,backup): document the retention/prune model and narrate the SSO + backup-reliability releases