Add remote storage backend

README.md

@@ -77,11 +77,35 @@ Current boundaries:
 
 - local development audio (`audio_dir` / `audio_files`) still works
 - `audio_dir`/`audio_files` and `audio_s3` are mutually exclusive
-- no real S3 backend or AWS SDK integration yet
+- real S3-compatible backend now exists in the storage adapter package
+- storage backend tests use fake storage and do not require live S3
 - no prepare-stage S3 list/download behavior yet
 - no archive-stage S3 upload/promotion behavior yet
 - no `current/manifest.json` or `current/run_id.txt` uploads yet
 
+## Remote Storage Backend
+
+Narratio includes an object-store backend layer for future prepare/archive work:
+
+- `List(ctx, prefix)`
+- `Download(ctx, key, localPath)`
+- `Upload(ctx, localPath, key, opts)`
+- `Exists(ctx, key)`
+
+Implemented backends:
+
+- fake storage backend for deterministic tests
+- S3-compatible backend built from `pipeline.storage.s3`
+
+Key invariant:
+
+- callers pass full bucket-relative object keys
+- storage backends do not prepend `root_prefix` and do not infer session/campaign paths
+
+Current boundary:
+
+- this backend layer is implemented but not yet wired into prepare-stage audio retrieval or archive-stage uploads
+
 ## Canonical Stage Order
 
 1. `prepare`