Add S3 backend and storage verification

src/artifactstore/storage/__init__.py

@@ -6,6 +6,7 @@ adding adapters never touches the registry or API layers.
 """
 
 from artifactstore.storage.backends.local import LocalBackend
+from artifactstore.storage.backends.s3 import S3Backend, S3BackendConfig
 from artifactstore.storage.registry import (
     clear as clear_backends,
 )
@@ -32,6 +33,8 @@ __all__ = [
     "DeletionResult",
     "LocalBackend",
     "ObjectNotFoundError",
+    "S3Backend",
+    "S3BackendConfig",
     "StorageBackend",
     "StorageObjectMetadata",
     "StorageReceipt",