Before the beta: remove the object metadata repair from the admin board #263

Follow-up to #255 (PR #262). The admin board's Object metadata repair (Check / Repair on the Monitoring tab, POST /api/storage/repair-metadata) is a one-off fix for an alpha migration: the bucket copy of the rename on 2026-10-20 dropped the R2 objects' custom metadata. Once production is repaired, nothing in the app needs it again. Remove it before the beta.

What goes

• apps/admin/src/objects.ts and apps/admin/test/objects.test.ts
• the route POST /api/storage/repair-metadata in apps/admin/src/index.ts
• the ObjectMetadata section in apps/admin/ui/src/Monitoring.tsx and repairMetadata / RepairPage in ui/src/api.ts
• docs: the row in features/admin.md, the bullet in features/monitoring.md, the repair sentence in infrastructure.md and known-issues.md

What stays

• The storage.repair audit action: rows already written must still render in the Audit tab. Keep the type and its label (or map unknown actions to a neutral label).

Before removing it

• Production has been repaired (Monitoring → Object metadata → Repair, then Check shows "all in order").
• Staging · Recreate from production copies objects with rclone, which adds mtime/btime and drops nothing from production's now-correct metadata. Either accept those two extra keys on staging, or give the clone an rclone --metadata-mapper that strips them. Without the repair button, a recreate can no longer clean them up.