Walk every done render and produce the .opus sibling in R2 for any key that doesn't have one. Returns immediately after spawning the background task — poll /v1/admin/articles/backfill-opus/status for progress.

Earlier revisions enqueued through audio_normalize_queue (an in-process asyncio.Queue). That worked for new renders because the render worker and normalize worker share a process, but not for this admin endpoint — the API and worker are separate Fly machines, so keys queued on the API never reach the worker's consumer. We now do the work directly on whichever machine receives the call (Docker image is identical so ffmpeg is there too).