Add object storage cleanup utilities

- Add ObjectStorageManager class with methods:
  - find_orphaned(): Find objects in storage not referenced in DB
  - cleanup_orphaned(): Delete orphaned objects with grace period
  - verify_integrity(): Check all referenced objects exist
  - stats(): Get storage statistics per schema/table

- Add schema.object_storage property to access utilities

Key features:
- Per-store cleanup (default + named stores)
- Delimiter-based S3 listing for Zarr efficiency
- Grace period to avoid race conditions with in-flight inserts
- Dry-run mode for safe previewing

Author: claude