BUILD backup-status-api.php — backup dir scanner for dashboard tile
New endpoint at backend/api/critical/backup-status-api.php. Action: get_backup_status. Scans backup dirs per BACKUP-REG.md groups and returns per group: name, level, newest file, timestamp, age_hours, age_label, health (ok/warn/stale), plus file list with individual timestamps. CRITICAL groups: platform-commands, platform-api, platform-tools-api, platform-root-infra, platform-governance. STANDARD: platform-html-pages, platform-debug. Health thresholds: ok=under 24h, warn=1-3d, stale=3d+. Full spec in Builder [02] inbox msg 363. Report back to inbox system=02 when live. Depends on T653 (BACKUP-REG.md registry update).