Add remote storage backend

internal/adapters/storage/object_store.go

@@ -0,0 +1,32 @@
+package storage
+
+import (
+	"context"
+	"time"
+)
+
+// ObjectStore is a remote object storage boundary used by future prepare/archive work.
+//
+// Key invariant:
+// callers pass full bucket-relative object keys. Backend implementations do not
+// infer Narratio session semantics and do not prepend root prefixes.
+type ObjectStore interface {
+	List(ctx context.Context, prefix string) ([]ObjectInfo, error)
+	Download(ctx context.Context, key, localPath string) error
+	Upload(ctx context.Context, localPath, key string, opts UploadOptions) (ObjectInfo, error)
+	Exists(ctx context.Context, key string) (bool, error)
+}
+
+// ObjectInfo describes one object in remote storage.
+type ObjectInfo struct {
+	Key          string
+	Size         int64
+	ETag         string
+	LastModified *time.Time
+}
+
+// UploadOptions configures optional object upload metadata.
+type UploadOptions struct {
+	Metadata    map[string]string
+	ContentType string
+}