Discard Brain Dump

Drop a half-uploaded take's server buffer when the user re-records.

The row and any stored audio are left alone — only the disposable part buffer is cleared.

recording_id says which take to drop. Without it the row's current take is assumed, which is wrong as soon as a second tab has moved the row on: the caller would clear a buffer that is still being filled. Callers know their own id, so they send it.