- EXPECTED_BACKENDS now includes onedrive, sftp, gcs, b2 (7 total)
- Added describe blocks for OneDrive, SFTP, GCS, Backblaze B2 field assertions
- 9 tests fail RED because registry.ts still has only 3 BackendType values
- Update BACKEND_REGISTRY[backend].length → .fields.length
- Update iteration to use BACKEND_REGISTRY[backend].fields
- Update field lookups (.find/.map) to go through .fields
- Add new test: each backend has displayName and description metadata (RED)