Add remote storage backend

docs/roadmap/narratio-s3-archive-implementation-plan.md

@@ -95,11 +95,14 @@ Implemented in repository:
   - campaign/session/run local work and spool path helpers
   - manifest run/path identity fields
 - examples and tests for the above foundations
+- remote storage backend layer:
+  - object-store abstraction with `List`, `Download`, `Upload`, and `Exists`
+  - fake storage backend for deterministic, no-network testing
+  - S3-compatible backend built from `storage.s3` config
+  - backend construction helper from resolved config
 
 Not implemented yet:
 
-- real S3 backend
-- AWS SDK integration
 - prepare-stage S3 list/download behavior
 - archive-stage S3 upload behavior
 - promotion uploads
@@ -849,9 +852,9 @@ Expected commit:
 Add archive storage path configuration
 ```
 
-### Storage Backend Interface and S3 Backend
+### Storage Backend Interface and S3 Backend (Implemented)
 
-Implement:
+Implemented:
 
 - storage backend interface
 - object metadata type
@@ -859,7 +862,7 @@ Implement:
 - real S3 backend using AWS SDK or existing project dependency policy
 - backend construction from config
 
-No stage behavior yet.
+No prepare/archive stage behavior yet.
 
 Expected commit:
 

docs/runbooks/s3-archive-foundations.md

@@ -12,11 +12,14 @@ This runbook documents the currently implemented storage/archive foundations and
 - promotion rule validation for safe relative paths
 - run ID generation and path/key helper functions
 - manifest run/path identity fields
+- remote storage backend layer:
+  - object-store interface (`List`, `Download`, `Upload`, `Exists`)
+  - fake backend for deterministic tests
+  - S3-compatible backend using AWS SDK v2
+  - config-based backend construction helper
 
 ## Not Implemented Yet
 
-- real S3 backend integration
-- AWS SDK wiring
 - prepare-stage S3 object listing or download
 - archive-stage S3 upload or promotion writes
 - writing `current/manifest.json` or `current/run_id.txt` in S3
@@ -29,4 +32,4 @@ This runbook documents the currently implemented storage/archive foundations and
 
 ## Next Implementation Target
 
-Build the storage backend layer that can list/download/upload S3 objects through a fake-tested interface, then wire prepare/archive stages to use that backend.
+Use the storage backend layer in prepare-stage session audio discovery/download flow, while preserving local audio input support.

docs/storage-backends.md

@@ -0,0 +1,58 @@
+# Storage Backends
+
+This document describes the currently implemented remote object storage backend layer used by Narratio, and its intended role in later prepare/archive work.
+
+## Implemented
+
+Remote object store abstraction:
+
+- `List(ctx, prefix)`
+- `Download(ctx, key, localPath)`
+- `Upload(ctx, localPath, key, opts)`
+- `Exists(ctx, key)`
+
+Object metadata model includes:
+
+- key
+- size
+- ETag (provider metadata only)
+- last modified time when available
+
+Backends:
+
+- fake storage backend for deterministic tests
+- S3-compatible backend implemented with AWS SDK for Go v2
+
+Construction:
+
+- config-based constructor builds S3 backend from `pipeline.storage.s3` values:
+  - bucket
+  - region
+  - endpoint
+  - force_path_style
+
+## Key Invariant
+
+- callers pass full bucket-relative object keys
+- storage backends do not prepend `root_prefix`
+- storage backends do not infer campaign/session/run paths
+
+S3 session/run key builders remain separate and continue to live outside backend implementations.
+
+## Security Boundary
+
+- do not store AWS credentials in Narratio config
+- AWS credentials are resolved through standard AWS SDK credential chains
+- AWS SDK-specific types remain isolated to the storage adapter package
+
+## Testing
+
+- fake storage tests cover list/download/upload/exists and error paths
+- S3 backend tests use injected fake S3 API clients
+- tests do not require live S3 services, AWS credentials, or network access
+
+## Not Implemented Yet
+
+- prepare-stage S3 object listing or downloads
+- archive-stage S3 uploads or promotion writes
+- writing `current/manifest.json` or `current/run_id.txt` to S3