Evict remote objects from every cache layer

DELETE /objects only removed the artifacts row, so mutable indexes (S3
index object + Redis TTL/ETag keys) kept being served. Clear all layers
and treat a trailing * as a prefix.

internal/storage/s3.go

@@ -99,6 +99,17 @@ func (s *S3) Stat(ctx context.Context, key string) (*minio.ObjectInfo, error) {
 	return &info, nil
 }
 
+// DeletePrefix removes every object whose key starts with prefix.
+func (s *S3) DeletePrefix(ctx context.Context, prefix string) error {
+	objects := s.client.ListObjects(ctx, s.bucket, minio.ListObjectsOptions{Prefix: prefix, Recursive: true})
+	for res := range s.client.RemoveObjects(ctx, s.bucket, objects, minio.RemoveObjectsOptions{}) {
+		if res.Err != nil {
+			return res.Err
+		}
+	}
+	return nil
+}
+
 // ListStaleObjects returns keys under prefix last modified before cutoff. Used
 // by the GC to reap abandoned staging objects (e.g. cancelled docker pushes).
 func (s *S3) ListStaleObjects(ctx context.Context, prefix string, cutoff time.Time) ([]string, error) {