Map s3 not-found to storage.ErrNotFound in Get and Stat (closes #129)

The Storer interface documents that Get and Stat return storage.ErrNotFound for a missing object. The file and rclone backends did; the s3 backend returned the raw SDK error, so callers testing for ErrNotFound behaved differently on s3.

S3Storer.Get and Stat now wrap ErrNotFound when the SDK reports a missing object and leave every other error untouched. The SDK reports a missing key two ways (NoSuchKey from Get, NotFound from Head); both are recognised in one helper, s3.IsNotFound, which HeadObject now also uses. The mapping lives in the storage package because internal/s3 cannot import it.

model: claude-opus-4-8 (implementation, review); claude-fable-5-1 (merge)

internal/storage/s3.go

@@ -38,14 +38,29 @@ func (s *S3Storer) PutWithProgress(
 }
 
 // Get retrieves data from the specified key.
+// Returns ErrNotFound if the object does not exist.
 func (s *S3Storer) Get(ctx context.Context, key string) (io.ReadCloser, error) {
-	return s.client.GetObject(ctx, key)
+	rc, err := s.client.GetObject(ctx, key)
+	if err != nil {
+		if s3.IsNotFound(err) {
+			return nil, fmt.Errorf("get %q: %w", key, ErrNotFound)
+		}
+
+		return nil, err
+	}
+
+	return rc, nil
 }
 
 // Stat returns metadata about an object without retrieving its contents.
+// Returns ErrNotFound if the object does not exist.
 func (s *S3Storer) Stat(ctx context.Context, key string) (*ObjectInfo, error) {
 	info, err := s.client.StatObject(ctx, key)
 	if err != nil {
+		if s3.IsNotFound(err) {
+			return nil, fmt.Errorf("stat %q: %w", key, ErrNotFound)
+		}
+
 		return nil, err
 	}