Update storage mirror task

Fix existing integrity errors preventing mirroring of attachments to the
secondary storage services.

lib/tasks/storage/storage.rb

@@ -46,7 +46,16 @@ def mirror
     count = mirrorable_blobs.count
 
     mirrorable_blobs.find_each.with_index do |blob, index|
-      mirror_service.mirror(blob.key, checksum: blob.checksum)
+      begin
+        mirror_service.mirror(blob.key, checksum: blob.checksum)
+      rescue ActiveStorage::IntegrityError => ex
+        raise ex unless @klass == FoiAttachment
+
+        # Fix for https://github.com/mysociety/alaveteli/issues/8181
+        attachment = FoiAttachment.joins(:file_blob).
+          find_by(active_storage_blobs: { id: blob })
+        FoiAttachmentMaskJob.perform_later(attachment) # this also mirrors
+      end
 
       print "#{prefix}: Mirrored #{index + 1}/#{count}"
     end