Unify external storage under OAS with content-addressed region

- All external storage uses OAS infrastructure
- Path-addressed: regular object@store (existing)
- Content-addressed: _content/ folder for <djblob@store>, <attach@store>
- ContentRegistry table for reference counting and GC
- ObjectRef returned for all external types (lazy access)
- Deduplication via SHA256 content hash

Co-authored-by: dimitri-yatsenko <dimitri@datajoint.com>

Author: claude