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.