Fixes two bugs reported in #5603:

1. store/attachment.go: ignore os.ErrNotExist when removing a local
   attachment file so that a missing file on disk (broken state from
   failed uploads) no longer blocks deletion of the DB record, allowing
   memos referencing corrupt attachments to be deleted normally.

2. memo_attachment_service.go: add nil guard on GetAttachment result
   before dereferencing it in SetMemoAttachments, preventing a nil
   pointer panic when an attachment UID no longer exists in the DB.